Getting the right length pushrods for my 408?

buy or make an adjustable checking pushrod. Get a lobe on base circle. Adj the length till you have .025" preload (hyd) or your cold lash is set (solids) with 1&1/2 threads showing. lock it solid & measure it & that's the length you need. I would highly recommend adjustable pushrods rather than custom made ones so you have some wiggle room. EDIT I'd suggest subbing in a light "checking" valve spring for this (Ace hardware has a good selection)