Lindsey, if you're asking me, yes, the Thrush glasspack. It has the straight-through flow described in the shop manual (don't know how important that is). But the Walker 21028 you mention has the outward appearance shown in the manual; mine has sloped ends rather than boxed ends. I know very little about these, only that the AutoZone search by vehicle make & model led me to the one I purchased, and it looks like the very old after-market muffler that was already on my truck.

DJ
'51 Chevy 3104 1/2-ton 216