It's always best to check your pushrod length with a pushrod length checker... Then you know you will have the correct length pushrods, though I don't really see anything in your setup that would require special length pushrods. Hopefully some of the chebbie guys will jump in here, but on any engine it is best to check pushrod length with the correct tool rather then guess.... A thinner head gasket would be a nominal increase in compression... You probably wouldn't want to be a lot higher then 9.5 with the cast iron heads and be able to run on pump gas. With aluminum heads, you can generally run about 1 point higher....