Wally, the 3701481 block was used in '53 in PG cars, but also used in '54 more widely. The stamped engine number on the pad just aft of the distributor (if the factory number is still there) will decode original service (letter prefix in '53, or letter-number suffix in '54).
Joe, there's something wrong with the numbers in your post - double check for typos? Casting 3701887 is a head casting, not a block casting number. I happen to have one on a '53 235 that's in my shop at the moment. I don't see '883 as a valid head casting number in my info. Also, the two block casting numbers to look for are 3701946 (as you posted) but the second is 3701483. Those are both low prssure, spray/splash oiling blocks. The block casting number will be low on the block, forward of the distributor.
Hope that helps.
Doug
