I wouldn't spend any serious $$ on the 557 short block. But, there are things you can do that will work with the next engine you have. A good Q-jet and intake manifold could be used on any Pontiac 350-455. Since you have 6x-8 heads, I'd recommend a 1972 cast iron intake. But, any '76-79" Pontiac 4-barrel intake will work.
Also, an Edelbrock "Performer Pontiac" intake will work just fine, on a mild to moderate 400. No performance increase over an iron intake. But, will keep carb slightly cooler. Will also accept a square bore carb, in case you don't wanna use a Q-jet.
A good Q-jet might give you the performance boost you desire. If not, then, as mentioned, numerically higher gears can make a big difference in acceleration. 3.23 & 3.42 gears were the top options, in late 2nd gens.
If this is still not enuff of a performance increase, there are Ram Air Exhaust manifolds, with 2 1/2" dual exhaust, with low restriction muffs. A word of warning here: MANY times, one or more of the exhaust manifold bolts will break off, rather than coming out, requiring the heads to be pulled & taken to a shop, to have the the broken bolts removed. So far, all these parts will work with your next engine.
If this still doesn't provide the power you seek, then a cam change might do it. The 6x-8 heads only provide aprox 7.6:1 compression. So, you need a "steep ramp" cam, like the Voodoo series. A 256 will increase low rpm response & torque more, and will work good with a stock converter.
But, a 262 will increase mid range & above, more than the 256. Might work better with a converter which stalls a little more than stock--maybe 2200-2400 stall. These Voodoo cams will require stronger valve springs.
If you wanna keep the stock valve springs, then an 068 clone cam is about as big as you can use. These 068 cams were used in lots of high compression 400's, and the lower compression 455HO engines.
Another way to increase power is to increase compression, by changing heads, or at least having your 6x-8 heads machined down some, to make the chambers smaller. But, keep in mind that increasing compression and performance, on a well used short block, COULD possibly cause blow-by & power loss.
I reckin it all depends on how much you are willing to spend, for more performance, and exactly how much of an increase will be enuff, for now.
Edited by ponyakr, 15 May 2017 - 09:14 AM.