All 1275 years (1967-1974) are good. The later years (12V tagged) seem to be more prone to being factory sleeved blocks. Sleeved blocks cannot be bored the usual oversizes due to the thin sleeve walls. However, the sleeved blocks make great candidates for a 1380 big bore. This is because the 1380 bore size cleans the sleeve right out of the block. Then you just have to hope that the block wasn't originally sleeved to correct a casting problem (usually not) in the bore area. Sleeving was most often used to correct sloppy boring of the block in the first place.
For me, the "Holy Grail" of in-line 1275 blocks, is the 12CD block with the thick oil pan flange and no hole below the center main bearing bore. The stiffest block of all the ones imported into the USA.
The earlier cylinder heads are better in the exhaust valve-pocket area. They do not have the large "heat-dissipation" boss that the later heads possessed. The non-emission and the early (1968-1970 or so) emission heads are cast with the better exhaust port. All 12V engines have the big boss ports.
So, to try and sum it up, the 12C_ engines have the better "package" of components. Discounting the head, then all 1275s are good. A grinder with a carbide ball will cut out the big exhaust guide boss in the 12V engines and transform a later head into an early head and correct that issue.
This should help you make your decision.
Mike Miller