inlet/outlet holes size
 

ok so im buying a thrush glasspack muffler.. but,
i know stock piping in a 99 blazer is 2.25, so would i have buy a 2.5" inlet/ outlet in order for the pipe to slide over the stock piping?

im not sure if that makes perfect sense but does anyone know what i mean?

You would still want a muffler for 2.25" pipe. The connections on either end of a muffler are made to slip over the outside of the pipe diameter they call out.