One piece rear mains on a 351-C are not worth the expense for many owners since it requires the engine block to be set up on a big mill and carefully bored to take the Windsor main seal. Careful builders then add J-B-Weld to the large unsupported area thus exposed in the block, but it seems to work OK without a JB seal support at least on street engines. Such machined blocks then will NOT ever accept a 2 piece seal again without a LOT of fussy work.
The problem shows up when the rear crank journal is undersized by almost any amount. With a 2-piece seal you trim the ends sightly to compensate for an 010"-030"-under crank. This is not an option with one piece seals and there are no known undersizes. On the good side, if you have a std sized crank, one-piece seals are available from Rock AutoParts in teflon which reduces drag a tiny bit. FWIW, I and thousands of other builders don't get leaks with stock 2 piece seals.