Hagerty Price Guide publishes values according to four grades.
1. Concours
#1 vehicles are the best in the world. Imagine the best vehicle, in the right colors, driving onto the lawn at the finest concours. Perfectly clean, the vehicle has been groomed down to the tire treads. Painted and chromed surfaces are mirror-like. Dust and dirt are banned, and materials used are correct and superbly fitted.
2. Excellent
#2 vehicles could win a local or regional show. They might even be former #1 vehicles that have been driven or have aged. Seasoned observers will have to look closely for flaws but will be able to find some. The paint, chrome, glass and finishes will all appear as excellent. The vehicle drives as a new vehicle of its era would.
3. Good
#3 vehicles drive and run well but are not used for daily transportation. The casual passerby will not find any visual flaws, but these vehicles might have some incorrect parts. #3 vehicles could possess some, but not all, of the issues of a #4 vehicle, but they will be balanced by other factors such as fresh paint or a new, correct interior.
4. Fair
#4 vehicles are daily drivers, with flaws visible to the naked eye. The chrome might have pitting, the windshield might be chipped and perhaps the body has a minor dent. Imperfect paintwork, split seams or a cracked dash might be present. No major parts are missing, but there might be non-stock additions. A #4 vehicle can also be a deteriorated restoration."
Taking all that into account, value is still subjective on these cars. Hagerty follows sales to assess value as well as desirability. I might be inclined to say if an Esprit with a 6cyl means -30% on the displayed value, it wouldn't be a far stretch to say if it is equipped with the 400 to +20 or 30%. However, as I stated above, usually there is a callout for all of the "value adjustments" and the only thing referenced is the -30% for the 6cyl.
That said, I don't think it would be unfair to ask for the determined Esprit value +20 or 30%, or to "consider it a Formula 400" and price it as such. Heck, maybe split the difference between the Formula 400 and Esprit.
A factory 400 is a factory 400, that's what counts. It's not like this was a 250ci that got swapped.
HOWEVER, you've stated it's not stock. To some buyers, that's a red flag. To others, it's OK but doesn't add any value, and to others, a couple grand more is a reasonable stretch on the value. Like I said, value is subjective.
Personally, I wouldn't pay extra for a built engine and I would actually be somewhat turned off by it. Any SBC or BOP can put down 600HP in stock form with nothing more than a cheap eBay turbo. ...but very few stay together for more than a few minutes after the dyno tune. ...if they make it off the dyno.
The only way you could convince me to buy your car at accepted retail value or more is if you could prove it was built to 800+ HP and you've detuned it to 600HP for reliability and longevity purposes. I'd even pay a little extra for that honestly because I know it's going to be a rock solid build laying down lazy horsepower, not one giving 110% and doing it's level best not to fly apart every time I drop the pedal. ...if that makes sense. Receipts proving you bought real (expensive) internals and not a bunch of Chineseium would go a long way, especially if it was built by a well known speed shop such as Butler. Unfortunately, "your word" means nothing to me in this area, and dealing with me you'd be better off NOT mentioning the engine build. Otherwise, if you want to ask a premium for that build, you've got to prove the value. But, maybe that's just me. I'm not a trusting person any more, some senior citizen ruined that for everyone this summer. (long story) ...but even if you could prove you bought top-of-the-line everything, there is still the issue of installation. Receipts from a speed shop would go a long way here. I can build an engine using nothing but the best of the best, and you wouldn't want it. ...neither would I because I'm not an engine builder, and if I overlooked something, or was a few thousandths off somewhere, it could be catastrophic. Not necessarily "going to blow up within 100 miles", but not necessarily going to make it to 20, 30 or 50k miles let alone over 100,000.
Make sense?
Speaking of "blowing up within 100 miles", seen this story?
That's a brand new hand built C8 engine. Not even 60 miles on the odometer, and there is NOTHING the owner could have done to cause the failure, the C8 computer limits RPM to below 4,000 during the 500 mile break-in period, so it's not like he red-lined a new engine. $#!T happens sometimes, and if a Certified GM Engine Builder can let one out the door that isn't 100%, what are my chances as a first time or even hobbyist builder to get my first one, or every one 100%?
I feel I need to say I'm not attacking you personally, your build, or anything. Just cluing you in to the different "mindsets" of potential buyers of Classic Cars and cars in general. You could buy a brand new car and have a $100,000 stereo professionally installed. You did NOT increase the value of the car. You actually decreased it, even if no panels were cut or holes drilled. Values are based on stock configurations. Deviate from those configurations and value becomes more subjective than ever.
Looks like you have a beautiful and well loved car sitting there. All I can really say is to find a price that is fair and reasonable using Hagerty and internet sales of comparable Esprit 350s as a guideline, and then stick to your price. Don't come off of it one Cent. Someone values the car just as highly as you do, and they will pay what they know it to be worth, maybe even a bit more given rarity, and I mean Firebirds in general. Far fewer of them than Camaros out there.
I'm sure the HP is fun. I had an '80 4spd N/A Z28 putting nearly 500 to the rear wheels. It ran low to mid 11s in the quarter (11.3 was my most consistent time) and it wheelied a little with the right tire at the right pressure. VERY FUN. Much more than a 17 year old me should have had.
...anyway, here are Hagerty's values, which are really just a guideline mainly for insurance purposes, but as I stated, they watch the market. Yours could easily be valued at much more, or a bit less. Location plays a huge roll. A rust free car is more valuable in the Northeast than it is in the Southeast or out West for example. The values listed are based on at least 500 recent sales for each vehicle. The Formula was like 576 sales and the Esprit was 546 or something, so there is real data behind these figures. Just take them as a guideline more than a definitive value though.