I can't tell you from the numbers you supply. Some one else should be able to. I'm also not sure how late of a car you can get a heritage cert for. As for building a 1275 GT being a closed back pre A+ block they are easy to fake. I took one in trade several years back but the crank had been ground 30/40 and the block bored 73.5. It did have the 11 stud 940 cyl. head and the awful AA style connecting rods. 

You give no indication of where you are located. That info may lead to additional help. Steve (CTR)

PS like your bike,  

hi all, I'm new on here! I have a 1978 1275GT and looking for an original engine, I'm looking at a engine the seller says it's an original 1275GT but can anybody confirm for me please it's a 1974 engine code 12h694aa head 12g940? I have looked on here at the code table but non the wiser lol thanks in advance