Should I get a new or used engine block to build a 351 w stroker?

any stroker engine requires ALOT of machine work (new or used block). other than the "normal" machine work for boring, line honing, etc., you will need to mock up the engine several times to check the rod clearance at the oil pan rails, cam bore, and more. almost always this has to be done by hand. there are some pre machined blocks out there but i haven't seen any 351ws. summit racing offers a prepped 302 block (347cid) and a gm 350 (383cid), might check with them or ford racing. otherwise, find a very good machine shop, or break out the die grinder.