The major consideration/difference is that the 1957 transmission is for an open driveshaft. Your 1949 3100 has an enclosed driveshaft "torque tube" rear end.
You can use your 1949 clutch housing (and clutch assembly) and 1949 transmission and your 1949 rear end - direct bolt-in.
Patrick recommends using a "short-shaft" water pump (that he sels for $$), but
this adaptation by Pre '68 Dave is far superior in many regards.
