Your transmission options are basically the same, running the Chevy six, GMC six, later Chevy six, or even most GM V-8's. The adapters to support putting modern automatics behind the earlier motors are not cheap, but they are out there.
My concern about the automatic in this case is the power the transmission will consume. You're choosing to limit power while pushing the demands on the drivetrain pretty far. A manual is going to allow more power to the ground.
The GM T-5 is a light duty transmission. While they seem to do fine in most inline applications and GM installed them in V-6 and 305 V-8 applications but did NOT in 350 V-8 applications. The reasoning is that the T-5 could not stand the additional torque.
The World Class T-5, which was used in Mustangs, is much tougher, but not an easy match to a GM bellhousing.
If you were not talking about a trailer I'd say the T-5 sounds like a good choice. Given that you want to pull a trailer I'd suggest looking at the A833 4 speed or the NV4500. They may be overkill, but the T-5 is marginal in a trailer pulling application.
Take your pick, risk failure or carry too much transmission.
