If your going with a flathead, the later models from 1949-53 were a better motor and had more HP. Your 1937 had a CID of 221 with 85HP and compression ratio of 7.50.1. Your later mod. 8BA ford ford and 8CM for mercury had lager CID of 239-255 and HP range from 110-125.

Whatever you use and there are alot of people who still like the flatties(including me), have the engine magnaflux for cracks as all flatheads had a tendency of cracked cylinder heads.