Type of fuel appears to be e85. But even if pump gas you are probably best off with a conservative compression ratio. Keep in mind boost psi and charge temp greatly affects what ratio you can run safely.
Pump gas under 8.5:1 with iron heads 9.5:1 with aluminum heads can be safe if you arent going for 30+ pounds of boost. At lower boost levels you may be able to run higher compression than this and many do.
E85 allows more compression. See quite a few on YBs forum pushing 10:1 or more with aluminum heads. Personally I'd rather turn compression down and run higher boost.
Transient response and spool up are a reason some opt for higher compression but smart turbine selection and cam choices can accomplish the same thing.
If you are doing a motor in the future and want to run your current signature engine with a turbo or two you have some options. Instead of buying dished pistons just find some large chamber 455 heads(115-124 cc) and put decent valves and stiffer springs and good gaskets. A single S475 for now and add a second one on the new motor. I suggest a good a/w intercooler now and re-use it later.
Your new motor...4.25 bore or thereabouts will hold gaskets better than larger bore. Stroke... 4" with 4.25 bore = 454... 4"x4.255=455... 4.25 x 4.25 = @482 tough call.
I think you should start a new thread ..."Build my motors"
Yes even with stockish heads a single s475 is going faster than your present combo.
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