If it has a 700r4, Transgo has a shift kit to make it work LIKE a full manual valvebody.
The shiftkit allows you to hold any gear to ANY rpm. But the trans will always start in 1st gear from a dead stiop. A true manual shift valvebody is in whatever gear the selctor says. Its possible with a TRUE manual shift valvebody to start in 3rd gear from a dead stop.
You will be fine with the shiftkit IMO. they also make a kit for teh 4l60e if your C-4 is of the the 94+ variety.