People may disagree with me, but I say stick with the 4spd case. The 3spd case isn't worth the hassle for a gearing change that isn't that noticable, if at all. A 4spd case is likely stronger than the 3spd and it will make less noise than the 3spd case on road (if that matters to you).
If you need gearing do the diffs, SM465, Orion or Toybox. More money, sure, but save your pennies and do it right.
I have the 4sp 3spd TC and really like it, My 2f came with a 4spd and TC that was broken in half, I used the 3spd TC because I did not have a 4spd TC. It is nice but I am not doing it on the 45 because I figure the $200 will toward something that will do more for the truck. Not really worth it.