Akhunter - nice to see you are still alive and have the GTO.
461CI is a good choice. I suggest you do box the frame to add some strength to handle the big increase in HP/Torque. The Currie 9" is a good choice, BUT, I would also put some thought into beefing up the rear suspension mounting points, especially the lower control arm mount. These mounting points were not intended for the HP/Torque your engine should be putting out. I went with a 9" housing/axles/brakes from QuickPerformance and then pieced mine together using a mix of manufacturers to get what I wanted.
Pics 1-3 show the factory lower control arm mount - not much of a bracket. I made my own templates and then brought them down to a fab shop to make them - it was just quicker for them to do it than me. I added a flat plate to the top & bottom, and a formed plate on the side where the bolt goes through. Then welded them on & painted.
I did the top as well, but I probably went overkill as I also added a roll formed angle on top of the factory crossmember to add strength to it. Picture #4
. Made the 2 cutouts to match the frame where the bolts go through on the upper control arm, and then welded an "ear" on the outsides that fit into the scallop so my upper control arm bolt goes through the "ear" and then through the factory mounting hole - effectively doubling the thickness of the mounting point where the bolt goes though. I also welded a tab on the other side opposite the "ear" to double the thickness of the opposing side of the control arm mount. I did not want to weld the angle to my crossmember, so I used a series of grade 8 bolts to attach it. The upper control arm bolts will additionally hold it into place.
TIP - to make sure I got the control arm bolt holes exactly correct, I found that a foam ear plug was the exact size as the bolt holes. I then put my fabbed pieces where they should be set, put some white spray paint on the end of the ear plug, and then put it through the factory bolt hole and pressed it onto the fabbed metal. This left a perfect circle on my piece and I then drilled it to bolt OD size. Then I put my pieces in place, ran a bolt though to align the holes, clamped up my parts, & welded - bolts fit nice and tight through the holes with no mismatching.
If you plate the bottom mount, the UMI control arm brace would probably be all you need to tie in the upper control arm mount with the beefed up lower control arm mount. Pic # 5 I have it set in place while I was trial fitting/welding everything. The UMI piece comes with all the needed bolts and it is adjustable so you can get it fitted correctly.
Transmission wise, I would suggest that you purchase the book "GM Automatic Overdrive Transmission" by Cliff Ruggles. This will help you make your choice and it shows what is involved in the installation.