Getting an R151F would save you from having to buy a $350 adapter and would likely be hooked to a 23 spline t-case, so there is the potential for cost savings depending on your goals and the need to rebuild the tranny. Also note that aftermarket (4.0, 4.7, 5.0) gears are the same price for 21 or 23 spline, so you could source a 21 spline case and add 23 spline gears to it to save money.

I agree that the lower first gear is unnecessary for Bill's application, but I think that it has more to do with the dual cases. The 3.4 isn't exactly a stump pulling engine.