[67_C-30]

I'm just saying this to save you a lot of trouble - but you are not going to be able to run 27 psi without lower compression pistons. The factory 20.3 : 1 CR of the 6.5 will take that much boost. At the very least, you need some thicker head gaskets to lower CR. Cometic makes them for these engines, but they are kinda pricey. Still cheaper than pistons though...

Good luck with your build. I put a turbo'd 6.2 in my '67 C-30 and at 12 psi, it will smoke all 4 rear tires in 2nd gear.