Turns out it a 76-78 , 400 block
I'm not disappointed tho, I read a couple of places that Ma Mopar continued to improve the 400 from it's inception in 71 to 78.
If it's true, and quality control increased, I think I might have a good block. Sonic testing is in order. Picking up the 400 next weekend
At this point it's going to be a 512
Can't pass up a balanced stroker kit from 440 source for $2250