Hey bj,
Welcome to the forum and you need no pardon. You will find a vast array of knowledge here so just ask any question anytime......keep it to welding...not women... :>))
I have done a lot of welding repairs on cast alum. transmissions. Also, these transmissions are newer ones from the '70s-90's, so I don't know about the older Powerglide if their composition is the same. I would tend to believe they should be. These were only repairs, so I just want you to get an idea of a start and maybe an idea to pursue your project. Most of my repairs were broken mounts, cracks, and straightening/reinforcing. With broken mounts, I clean thoroughly and wire brush, and grind a bevel on the entire mating area. I find that a preheat works the best on the welded area, but I take a precaution to heatsink all surrounding areas with steel plates. Also, I use steel plates clamped to the mount and body area to maintain alignment. After setting up and cleaning thoroughly, I preheat to 350-400f using a surface temp. gauge. I MIG weld with .035, 5356, with Argon at 32cfh and heat at 140A & WF@600ipm.
Since your project is quite extensive, I would suggest that your best results would be TIG, and someone who is very experienced. Your best approach would be to do small sections at a time to try to maintain a lower overall heat saturation on the entire unit. You may find using some 1/2" alum. angle to reinforce your joining of the two units. Aluminum is not difficult if you proceed with careful forethought. I would definitely do some hard practice on some scrap material or a junker transmission to work out any bugs and allow yourselves the opportunity to examine your efforts and parameters.
This sounds like an interesting project and I know others here would be interested in your results and if success is obtained. Please keep us up to date on your progress.....Good Luck...Denny