It should have 308 on it, by that time the HP casting had all but finished. I was mixing my dates up too so it could be any HT or even early HG with that date code. I was for some reason thinking May 1969 which is almost certainly a Brougham 308.

If not decked the engine number will be 308Hxxxxx.

In the 80's Gem used whatever block in anything when you exchanged.