Yeah, you'd practically need to find a donor truck to get all the parts to do a 4L60E conversion without it destroying you nickel-and-dime style, and it would have to be a 1993-1994 only, with same engine you've got, so you could use the engine harness and PCM from the donor.
You're definitely right that 700R4 (4L60) would be the way to go since you're a '91. You'd still need to setup the torque converter lockup control; to be perfectly honest I've never looked into it enough to know what's needed. Probably the engine side wiring harness so you'd have the minimal amount of wiring to it (or dig hard into factory wiring diagrams to do it yourself) and I'd guess the automatic compatible ECM or at least the memcal from the ECM. Again obviously you'd want to stick to same engine and very close to same year.
The only appreciable difference between a 700R4 (4L60) and the 4L60E would be the electronic control; you don't really gain anything on a daily driver by the extra work of going to the 4L60E. It has some advantage to a person wanting to do custom tuning as they can have better control over it through tuning. Otherwise the transmissions are from the same family and are close enough mechanically that it's not like one is stronger than the other; they're both light duty automatics.
I say go 700R4 and just be sure you really, really read up on proper adjustment of the throttle valve cable so you don't roast it right away. Totally critical to the behavior and lifespan of the transmission.
Richard