4th Generation starter problem

Hey everyone, new member here. Hopefully someone here can help me with my problem.

I recently came into possession of a 2000 chevy blazer with the 4.3 liter V6. I was told the only thing wrong with it was the starter motor needed to be shimed. I got it home and after working on it for a while I realized that they had bought the wrong replacement starter. After more research I found out that the starter on the blazer was correct for that year. I then found out that the motor had been rebuilt but it was not known what kind of block was used. Nor do I have access to the starter that was taken off in order to compare. I have looked all over and can not find the block casting numbers. Can anyone give me more information or better pictures of where to look to find these numbers? I would greatly appreciate it.

Thanks,
Lurch