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.
Well, ok- but you left the cost of purchasing the R151 out of the equation! It really is pretty much internally the same as an R150 (1st gear ratio excepted), which he has, except for the tailpiece. I'd bet a decent R151 will cost more than his V6 adapter. As far as a regeared tcase, net/net I agree, the cost is the same IF he's gonna regear immediately. FWIW, I am running a 21 spline rear case, but it is Marlin's total spline design, with which they have had no failures. Not worried.
I dunno, my 3.4 pulls stumps pretty well. The superfluity of the lower first gear probably has to do with 5.29s more than dual cases! <img src="/forums/images/graemlins/lol.gif" alt="" /> It could pull 4.88s easily (but I'm only running 35s).
One enduring advantage of the R151 is that my driveline with dual cases is that much shorter than it would be with an R150 and adapter. I'm not in TJ land with that concern, but it helps.
[edit] I've often wondered what would be involved in retrofitting an R151 tailpiece to an R150... <img src="/forums/images/graemlins/drunk.gif" alt="" />
