You don't need them for a 351W if you plan on 800HP or less. If you are going for 800+ then there are several manufactures like CAT marketed as "Pro-Line" from flatlanderracing.com and others like Milodon, Pro-Gram, and others I can't think of right now. Use a guirdle and ARP studs for the 351W and be good for 800HP. 460s have the same 800HP requirement for going to 4 bolt caps because they use 1/2" main bolts/studs like the 351 W/C and that is the weak link but a 351W has less rotational weight pushing on those caps than a 460. The 351W caps are smaller than the 460s but they equal out as far as HP capability is concerned because of the 1/2" mains studs/bolts. Be sure you really need 4 bolt caps before you spend $500-$600 on a stock thin bore block to convert to 4 bolt mains. Very few can be punched over .090" over but some can go .120" while others of the same '69-'71 year can only go .060". Sonic check it first to make sure it doesn't have bad core shift. Most 351W blocks did have bad core shift unfortunately.