In times past a fair number of makers produced adaptors mating up the flatty transmissions to later model motors. The adaptors still come up for sale often on e-bay, and once if memory serves, I saw a unit to match something big, like an MEL to a flathead trans.
What are some of your recollections about how the transmissions held up to the increased horsepowers? I am using a 46 3 speed in the 292, and have been treating it like a newborn, over concern about the transmission, even though the trans is like new. The other day I did do a 0 to 4500 rpm burnout just to see what would happen.