things that effect pushrod length, CAM BASE CIRCLE, block deck, heads milled, which heads, head gasket thinkness, rockers, valve stem height, lifters. all effect pushrod length. every build is different and every build requires builder to check pushrod length for correct geometry. most mild aftermarket flat tappet hydr. cams are nothing more than a regrind of a factory cam and in order to gain lift and duration the cam base circle must be made smaller. . example factory cam has .300 lobe lift and a BC of 1.5 and with 1.6 rockers net lift is .480. To change said cam to .520 lift the base circle must be ground smaller by .025 giving a lobe lift of .325 and net lift of .520 and a base circle of 1.475 that only accounts for lift , if one is adding duration the base circle has to be further ground to have material on the lobe to further grind the lobe profile. I have seen base circles shrunk as much as .200
this should be over in tech I would think. .