"...68 Pontiac 400 with #16 heads. Since I'm starting with a 350hp engine stock, could I just put in a mild cam, long tube headers with a little bit bigger diamter exhaust, and a 750cfm edelbrock, I should be in that mid 400hp range correct?..."
Don't think you can squeeze 450hp out of a 400 with a "mild cam, long tube headers with a little bit bigger diamter exhaust, and a 750cfm edelbrock," :no
For one thing, the #16 heads have too much compression for SAFE use with pump gas. Most Pontiac engine builders recommend 9.5:1 max--9.0 to 9.25:1 safer. OR, you can run a cam with lots of advertised duration, like a RA4 grind or larger, and make sure your total timing never gets high enuff to cause detonation.
You can run the #16 heads, with dish pistons, to reduce comp. But, when the comp is reduced, it's harder to make that 450hp you want. Yes, it can be done. And you can go with a stall converter and more rear end gear, to make your car "feel" like it has more power. I reckin you could call it more "seat of the pants" power.
BUT, as has been said many times, on this and other Pontiac forums, it is far easier to make 400+hp and 500ft lbs of torque, with a 455 or bigger, long stroke engine. The cheaper balanced cast crank stroker rotating assemblies are between about $1500 & $1800.
The best cheap iron heads for this stroker engine seem to be the 6x-8 heads. You can sell your #16 heads for plenty, to buy a good set of 6x cores, and pay for some of the machine work. There always seem to be plenty of 6x-8 heads for sale, in all stages and conditions, including ported and ready to bolt on.
"...it's going in my 78 Trans Am."
Keep in mind that the '68 block only has 2 motor mount bolt holes, which will require adapters to fit the 3-hole mounts in your '78 TA. Some have posted that the adapters don't work too well sometimes and require mods and fab work.
"... a 750cfm edelbrock..."
The only Edelrock carb I'd recommend is one of their Q-jets. But, I'd really rather have a PROPERLY REBUILT '76-'79 800 Q-jet, with the APT feature. Everyday & SMI have a good rep for these Q-jets. Everyday has one listed on their website, if you don't need the TH350 kickdown cable hook-up.
FrankTinCA just bought one, with the TH350 kickdown cable type throttle lever, from Everyday.