340 build

OK A BODY PEEPS...building a 1971 340, swap meet find carb to pan in a crate, ran the #'s on the parts, everything is correct so far...bolting down the rocker shafts and have rocker screws backed all the way off and im still opening the valves before i ever get the rocker shaft tight??? different length pushrods??? 1st mopar dont know what interchanges with what? any help please thx