Engine adaptor is available for an AT. The rear end needs to be changed to an open driveline. Since the rear must be changed it would make more sense and be more economical to get an appropriate ratio and use a less expensive tranny like a TH350 and get the same results. You'll need an oil cooler and lines, shift linkage, crossmember, mounts, etc. A T5 manual 5 speed can also be used.
You truck would be a 3100 series as you've described it and the standard engine was a 216, not a 261 unless it has been changed. If so then your OK with the transmission change. That is a pretty good engine.
