Is the three speed transmission a standard or automatic? It would be way easier and probably cheaper to use an overdrive transmission and leave the rear end in place. With either the old Borgwarner standard 3sp OD trans or a 700R4/2004R automatic OD you keep the 4.57 for pulling/hauling but have a 3.07 final gear when in OD. Best of both worlds. Many have good luck with the T5 OD set up but my uses have always been too much for this light duty unit. Have had very good service from the TKO 500/600 but they are pricey. The NV 3500 or 4500 are another trans option.

Evan