340 with 302 cast head for pickup

So do you have the exact model of piston? If flat-top as in the original post, your computed 13.5:1 static CR is way off; more like 10-10.5:1 with the stock 64 cc 302 head chamber, standard 340 pistons, .053 head gasket, and any sort of eyebrows in the pistons. It's hard to call this one without better piston info.....you need to help us to help you.

Are these some forged units or what? Domed or flat-top? Are they truly stock factory pistons? Speed Pro? Sealed Power? Original 10.5's weren't really 10.5 CR in the engine.....

That big cam is going to lower the DCR a lot. Agreed that it is not matched to the drive or manifolds.