Minor detail!
A rebuilt 4l80 with shift kit will do fine behind a mech injected engine and standalone controller. You might not need a shift kit if your controller can raise pressures and shift point to the appropriate spots.
You might be able to pull a 6l85 from a wrecked 08+ with a 6.0L 3/4ton (not sure when they started putting 6 speeds behind the gassers), or i think there are 6 speeds behind the 1/2 ton gassers too. It would still be pricey but surely better than new prices.