If you insist on keeping 6 cylinders I would go with a 292 or 302 GMC. A 261 might be ok but would be a little light on power when towing. However, if you kept the bypass thermostat in place you wouldn't blow it up if you lug it.
I would go with a T5 transmission if you use a 6 and keep about 4.11 or 3.90 gears. Then you get overdrive to run 70 with but enough grunt to pull the trailer off the line.
That's a pretty heavy trailer, though, so if you think you would want a V8 go with it. Gas milage wouldn't be that much different. I would use an NV4500 transmission but that's because I hate automatics, especially when towing.
