I know nothing about the Top Loader, other than it is a 4 speed that came in the original Cobra.
I choose the TKO600R, because I wanted a new transmission (I don't think they make top loaders anymore, do they?) and a 5 speed.
Here is some interesting info On the TKO's:
Tremec TKO Page
TKO Specs
Graph Of Revs TKO 600R (.82 OD) Vs Vette
How to choose the speedometer gear:
Tremec-Tire-Revs-Per-Mile-Calculator.xls
and then choose your gear:
Gears Part Numbers
.