I think you may be confused on a couple things. A 351W with a C6 would have come from some larger car line in '73. Never offered in trucks then. All 351W's use a 28oz imbalance flexplate. Only post '81 302's used the 50oz. Do not attempt to use a 50oz plate on your engine, it will tear it up in a hurry. A late 80's-early 90's AOD would be what you'll want to look for. They have the best upgrades, but be sure you don't get the later AOD-E electronic version. It's designed to work with the engine management computer.
52 Mercury M-1
among other things.