I did something Similar this past spring, and its still my runner....Blew up my 1993 bubble back 115, and already had a 1984 freshly rebuilt that came out real nice. I needed to get back on the water.....
While putting the '84 block on the existing setup...I Notice the block didnt sit fully flat onto the adapter plate (lower unit on). It rocked back and forth a bit. and it was binding when I rotated the flywheel. I removed the lower, to reinstall, and it wouldnt go in all the way.
If you search my posts, you will see that between those years there is a difference in the Crank portion that accepts that drive shaft, or vice-versa. I had to cut a bit off the top of the drive shaft, and grind in the "shoulder step-area" at the top of the driveshaft.
Only then, did the head sit flat on the adapter, and not bind the shaft...I knew I took enough off, when I couldnt feel the shaft binding anymore when turning the flywheel and/or prop.
I dont know if this applies to the 1991 also, But its just about identical to the '93 so I imagine it would.