I see it the other way around they unlock 80 hp.
The stock 340 x heads are good for about 400 hp gross with a reasonable build. So say a stock 340 does make 275 hp gross so there's 125 hp waiting to be unleashed what's mainly holding it back, Cam, Exhaust, Carb etc.. why wouldn't they add decent power?
Why do you think LS/gen 3 hemis make 150 hp gains with cam and headers, it's the potential not being used, even a 318 has like a 150 untapped hp out of stock heads.
Same with stock 1.88 j heads 375 hp should be no problem, what's holding it back Cam, Exhaust etc..
Now with compression ratio it's just gonna raise or lower the entire curve a few percent for each point you move from. If you think of 10:1 as the middle of 7-13:1, I say with the appropriate intake/exhaust a 10:1 240 cam 340 with stock 1.88 j heads should make around 375 hp.
Op's engine falls a bit short of that but by not much.