I had started to wonder that. Every combo I've had in the car is supposed to be internally balanced. I've been concerned cuz this block has an 'E' stapmed on the pad and is a 71, not original to the car but was in the car when I got it with 43k in it. Engine has been rebuilt by a cheap house back in the day, had a reman tag on it.
It'd be sad that if it's always had a cast crack in it and in 21 years of ownership I didn't realize it.
This would be the first one I've mismatched, do they normally act up in only 1 rpm range?