Originally Posted by cowlitzcoach
GM didn't come out with the 6.0L until the 1998 model year in most applications.
In 1990 it would have been a 305 or 350 small block or a 454 big block. The transmission would have been the TH400 or perhaps the TH700R4/4L60.
If it really is a 6.0L it would be the 4L80E or the 6L80 or 6L90. If it is the 8.1L it would be the 4L85E or the 6L80/6L90 or the 8L90.
You forgot about at least one other big block............
It could be a 366 (6.0L) big block Chevy, used in school buses up to the mid 1990's.