I have seen a 19 with a Ford 302 V8 in it. It was original equipment, and was the model "888" Mercruiser. It had through-hull exhaust (not original), and sounded very sensual.
The sales sheets show the '74 Seafari was offered with the 888 Mercruiser (Ford 302), as well as the 225 OMC and 245 OMC (Chevy SB 307ci and 350ci V8).
Therefore a Ford 302 WILL fit into a Seafari. The question in my mind would be deck height if the original motor was a 140. With original rear scuppers the rear deck would be a bit wet.
Here is the sales spec sheet from elsewhere on this site.
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Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury