If you don't have a GMC, the prices are pretty good now and lots of them available. We've only had ours for about 18 months and love it and the community.If your into cars or/camping/traveling, and can afford it, go for it IMHO.
The 4l80e is a modern electronic version of the th400 with a 25% overdrive. I was looking for a complete package with low end pull as well as low rev's at cruising speed for the enhanced mileage. The motor can be bolted to the original th425 with a simple adapter but you sacrifice either hi or low speed performance without the additional gearing. Of course doing it the th425 is a much simpler and less expensive way and probably within many peoples ability. And I'm sure will give excellent results. I've never followed the norm, always looking for a better (hopefully) way and not being afraid to give it a try. Maybe off bubble a little , who knows. Anyway, it's the approach I've taken and time will tell if it's right.
I'm running a 215/85/16 tires so the rpm's are slightly off the more popular 225/75/16.

215/75/16 tire; 225/75/16 tire

55 mph 1869 rpm in OD, 2492 in direct 55 mph 1939 rpm in OD 2586 in direct
60 2039 2719 60 2116 2821
65 2209 2946 65 2292 3056
70 2379 3172 70 2468 3291