Thanks for the input, Tim. If it weren't too late, I would heed your advice and go with the NP435 and get the low-range 1st gear. I have owned the NV3500 for almost a year, though, and it's one reason I decided to go with a Dodge V8 instead of SBC. My NV3500 came out of a '99 Ram 1500 and has about 40K miles on it. It has the integrated bellhousing that will bolt up to the 5.2L V8.
In order to get a better crawl ratio, I may end up putting a 4:1 TeraLow kit into the NP231. But I'll worry about that later.