[Brad54]

Captain is right about the durability of the trans.
I went through all this decisioneering for my Suburban. Like your GMC, I had a Dana 44 in mine, which gives a lot more options.
I made a chart that showed the gear splits with the Granny 4spd (yours), and a 4spd OD manual trans (since I had one and it was a bolt-in).
I figured effective ratios with the trans gears and different rear end gears, with an eye to final drive ratio at highway speeds.

With the Granny 4-speed, you could get 2.90 or 3.08 or 3.23 gears in the rear and still leave the line, with the 4spd OD I have (.72:1 final drive), I went with 3.42 gears.

I like jack-rabbit starting and 3.42 gears are still enough to pull a light trailer, so that was why I went with my choice.

5-speeds are a great choice too, but I didn't have one, and I don't know what's required to put them in our trucks as far as bellhousings and new cross members. The 4spd OD was a bolt-in for my '62 torsion-bar/X-frame truck with the bellhousing engine mounts.

-Brad