GMBs,
It sure seems like the T-5 that you have would work great, as is. In fact, I would think that with the 4.57 rear gear the 3.76 first gear would be preferable to the 4.03 first gear. Our 64 C-20 has the 4.57 gear in it and our 5 speed has a 2.95 first gear and the only time I wish I had more "gear" is when I'm pulling a trailer. I guess it all depends on how you are going to use the truck.
Good luck!
Ron