A bit more on 351-C main seals. The original Ford rope seal was used 1969-'71, then supplanted by a 2-piece neoprene seal from mid-1972 until the end of production. The high-friction rope seal was held in place by a steel pin pressed into the #5 main cap. If a neoprene seal is substituted, the pin should be tapped thru its hole and removed, and a spot of RTV used to plug the resulting hole. Otherwise, a main seal leak may result, according to Ford.
Finally, in 1980 a one-piece rear main seal in both neoprene rubber and PTFE (teflon) was introduced for the 351-W that also fits the 351-C, but precision machining of the #5 Cleveland cap and block must be done to slightly enlarge the seal recess. The one-piece rear main seal is NOT a drop-in substitute! Once machined, a 351-C exposes a shallow, unsupported area that can be filled with JB-Weld to better support the one-piece seal under racing stresses.
To install the one-piece rear seal, the ZF, clutch and flywheel must be removed for access; the 2-piece seal can be tapped into an installed 351-C engine using a non-metallic pusher-bar, NOT a screwdriver which may nick the crank.
Note also, a 351-W front main seal also fits the 351-C crank as a substitute, but the Windsor front seal has no external seating lip, so extreme care must be used during installation in a Cleveland. Its absurdly easy to push the lip-less Windsor seal past 'flush' and clear thru into the block extension's front cover recess!