I'd invest in a pushrod length tool and check it....my last 351 was built by a guy who didn't check, he put rods in that were too long...you can guess what happened after that.
Sorry i don't have a better answer....i'm actually doing the same thing right now putting 351 heads on my 302 and i've gotta get a checker to find out the right size. Then plunk down a bunch of cash for hardened rods.