I did a little more sleuthing because I am a curious bugger.
When I blow up your pics the best I can figure is the head says 5X, and you may not find a 5X head listed, there is diff info on diff sites, there are also oddities like @72-73 Pontiac got nailed by the EPA and had to redo engines and codes and they also had a strike, some engines ended up with a ZE code and you'll never find it, anyway... it could also be a 5I or 51 all the same head.
The engine may have a build date of 1974 but it was built for the 75 models, that head looks original as does the entire block.
I believe what you have is a 1975 455 and I would suspect the block says YU or YW because it's an auto, it has the 067 cam and 5X heads which are 7.5-1 comp, don't worry about too much power in it's present state it's like 200, torque will still be ok, that is actually a good head if you had it milled and rebuilt maybe to 9-1 and pop in the 068 cam you will have a 300-350hp terror. When you factor in the cost of rebuilt/milled heads vs edelbrock it should be much cheaper, most Pontiac heads are desirable except for the post 71 lower comp which may be ok if they are 8.5-1, but the 7.5-1 was just Pontiac trying to get by the emissions and they don't go.
