If your 340 has a true 12.5 Scr, it's gonna need a pretty late intake-closing event to get the pressure under 200psi. You cannot run that high with iron heads anywhere near sea-level, on pump gas. You're just fooling yourself talking about distributors and carbs.
>In your case;
Since the engine seems to be out and the heads not yet on;
The way I see it, is you got a choice, either; reduce the cylinder pressure now, or install a high-quality water-injection system later. I would have recommended alloy heads but I doubt you can get the pressure under 200 with any of them, without changing or modifying the pistons.
Some pistons have enough meat in the crowns that you can mill some off.
Sometimes you just gotta back up to go ahead; it happens.
>Finally
IMO-1
If you do have a really big cam, you MAY need a matching really big convertor and WILL NEED really big gears. If you don't match things up you're gonna be really unhappy. If your combo currently has a factory stall convertor and say 3.23gears, that's NOT gonna match up to a 292 cam, and so, it'll cost you MORE to change all that, than to get rid of those 12.5 pistons.
IMO-2
with iron heads and a tightQuench under 040, about the most pressure you can run on pumpgas and without water-injection, seems to be 165psi with; full timing, a typical stall, and typical street gears, at sealevel. You can try it at 170, but be prepared to make changes; so build the combo in such a way that you can drop pressure with a thicker gasket, but do not exceed 050 quench, cuz then you are likely to get back into detonation. .
IMO-3
Vacuum Secondary Holleys suck, lol; they're always too slow. When I floor it, I expect chit to happen, like instantly!
If I want slow, I just won't floor it.
>In my case;
I solved the whole too-high pressure issue by running closed chamber Alloy heads on a slightly higher than Zero-deck 367. With those, I have run the cylinder pressure up to
[email protected], and she still burned 87E10 Canadian gas.
In my combo I can change the pressure about 7psi/4degrees of cam-timing.
I built mine with an 028 gasket/028 quench, and it ran up to a tic over 200psi with no detectable detonation, even on 87E10.
But the 028 gasket I used, couldn't take it. So I took the engine apart, decked it to pop the pistons out some, installed the 039 FelPros, and this increased the Q to 032. It's been that way now thru two more cams and 100,000 plus miles. Oh and BTW, still on the same plugs, lol