[Coley]

Ok....so what I am hearing is that you are on a bit of budget with all of this...ok, thats a good start point.
So you have 2 main problems....1) one is that the existing diff in your truck is geared to low to be useful ....and 2) you currently don't have a transmission for your truck.
If you going to do an LS swap get ready for putting out money well into 4 figures (thousands) to get it going....do you have the budget and time for this?
If not...then consider finding another TH350 transmission or maybe another 700R series if the driveshaft, etc has already been modified for the 700 trans....there are lots out there and you might be able to find a quality used one without too much trouble.
Then I would look to find a correct '72 stock differential with the 3.07:1 ratio. I you go to any local wrecking yard they might have a truck in there and you could get that diff for a few hundred bucks.
If you needed to regain some money, then you could resell the low geared one you have in there.
There are enough of these trucks out there as parts trucks, wrecking yards etc that finding a diff shouldn't be too difficult if you do some searching and they aren't all that tough to put in.
my two bits.

Coley