JAC prefix equates to a 1951 216 ci. Block casting number should be 3835497, also look behind the starter or a bit forward on that side for date code. Like B 13 1, B is 2nd month Feb, 13 day of the month, 1 is year like 1951 etc.
Hope this helps.

It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed