The T98 has a granny 1st gear so a taller rear gear should work reasonably well. You'll probably still have a better ratio, 1st gear ratio x axle ratio, than you would with a standard transmission and, say, a 4.11 so towing may still be ok if that's what you want to do. However, the shift from 1 to 2 will be quite large and you may or may not like that depending on how you want to drive the truck. If you want a more performance feel, then this is probably not the route you want to take but it should work ok to get cruising rpm down while still having the ability to tow.
With a T5 you get the benefits of a much more modern transmission, more sporty shifting, and depending on which T5 you get, you can run around a 4.80 rear for towing and still have a better setup for the highway for cruising than you will have with the T98. 1st gear may not be as good as the T98 but it will be better than the 3 speeds of the period. To use a T5, you may need to do the transmission and rear end. If you already have a taller rear gear, then overdrive could be too much. So you have to factor that into the total cost.
Lawrenceville, GA