Okay I'm going to fall inbetween the other two answers.
First, I have 3.08 rear gears in my 1/2-ton Suburban, and a stock SM420 (granny gear 4-speed). Its very drivable that way, reasonable top speed, pulls well in general use.
I recently took the trailer hitch off that truck because its not much good pulling a trailer (and it looked ugly).
Before OD transmissions became the popular choice they are today, swapping to a car type Saginaw 4-speed was popular. The real advantage over a granny gear 4-speed or your 3-speed is the spread between the gears.
When you shift your 3-speed you loose so many RPM that you drop below the real power band of your motor. With the car type 4-speeds your RPM drop between gears stays in the power band better.
All the documentation I've seen on this was based on a Saginaw not your Muncie. I don't recall the ratios on an M-20, but I would expect the end result to be similar.
There is more than one model of T-5 out there. It offers the advantage of overdrive, which might be nice, but they are not nearly as stout as the Muncie. Since you're running a six, unless you are going to modify it heavily that may not matter, but it would be another thing to be aware of. Especially if you are thinking someday you'll go V-8 or highly modified six.
Another thing to think about is the shifter. Any tail shaft mounted shifter is likely to poke through the floor close to or under the seat. There is a shifter available from Patrick's for the Saginaw that deals with that problem. Given the differences between the Saginaw and the Muncie I would not expect it to work on the Muncie.
I've considered the Saginaw swap and always conclude the T-5 swap makes more sense. But if I had a Saginaw or Muncie M-20/M-22 or a Super T-10 laying around most likely it would already be installed in my Suburban.
I do have a Doug Nash 4+3 which is a Super T-10 with an overdrive. It isn't a popular transmission for this kind of swap, apparently because of the computer controlled OD. That transmission may someday end up on one of my trucks, but I wouldn't suggest putting it on your list.
