The AW4 has been compared rather favourably to the THM700R4. Also, the early THM700R4s were crap, and wouldn't hold up to the 2.8L V6 until bloody 1990 or so...
The THM700R4 became the 4L60, IIRC - one of those should also work for your application, but the 4L60-E is fully electronically controlled, and you'd have to move electronics over with the thing and figure out how to get them to talk to your system.
However, I'm inclined to think that the THM700R4/4L60 isn't really an upgrade over the AW4, having been inside both boxes. If you get a decent 700/4L60, the difference is nominal at best. If you really want to upgrade, look for THM350/THM400 or A727 - a Powerglide if you can find one! Those boxes are damned near bombproof, but lack overdrive (the Powerglide is a two-speed box that doesn't really even have a low gear, and will probably require a divorced transfer case mount...)
Just because there aren't a lot of upgrades available, doesn't mean that it's an "inferior" box. Look how (relatively) few upgrades there are for the AMC six - but they'll keep stomping along through anything you care to hand their way, and ask for more. Most of the internal "upgrades" would either be purpose-built parts or just happen to fit from other applications...