Yeah I've always thought 2.5" was a little whispy.  I usually do 2.25" or even 2".  I have a '91 Celica that actually performs better on the stock 1.75" than the larger exhaust.

Anyways have fun and post up whatever you do.  I love 1st gen Pickups especially '79s and '80s they are my absolute favorite Toyota 4x4s.