HU suffix can be one of three installations:
65 COR 327 350HP 1x4bbl L-79 4speed with a/c
67 IMP 327 275hp powerglide 1x4bbl L-30
69 IMP 350 255hp powerglide 1x4bbl LM-1

The block casting 3932388 is listed as being a "68-79 350" car OR truck so it appears to be the 69IMP source. Could be a 4 bolt main block