My 318 Build with questions

I am coming up with around 8.3:1 with Felpro 1008 head gaskets (.039" thick). The 16 cc number looks a mite low.... which would make the CR compute out a bit higher than reality. Do your pistons have valve reliefs or as they flat on the tops? Are these the stock pistons that were in there? If flat tops, the 16 cc number makes more sense.

BTW, I never answered the extended collectors question. You just weld another 6" or 8" or 10" onto the existing collectors at the existing collector diameter. That is done to extend the headers' effective RPM lower and give some more low end torque. I did it once upon a time and it did indeed help the low end. The engine had large breathing heads, intake and headers, but was cammed for low RPM, so I cannot say if that combo made the collector extensions have more effect.... or not.

And SCR is the static compression ratio.... the classic CR number like we are kicking around here. DCR is dynamic compression ratio and takes into account the fact the intake valve closes part way up on the compression stroke and so the effective stoke is shortened; that lowers what you could call the 'effective' CR, or what the engine really sees in terms of operating CR. It is derived from SCR and the cam timing, and has a very real effect on actual torque; for example, all else being equal, longer duration cams lower DCR and thus cause the classic drop in low RPM torque with a big cam. DCR is also computed to see if a SCR and cam combination pushes too close to the point where detonation can easily occur.