Welcome to the site from WA State.
Look into the TKX, its the latest/greatest out there, granted not cheap but is supposed to take a fair amount of HP. That said, you are not the first guy to go down this road and it can be done with a number of transmissions. It sounds like you want a road race car more than a drag car so that will be a little easier on the trans, meaning you probably have more selection than you think. Also, just out racingjunk.com, you might find some beefy race build transmissions there that might work. As far as the adapters, they exist you just have to do some research.
On the 4 link side, I think there is someone who makes a bolt/weld in system for E bodies but I made my own using a generic kit from Art Morrison (local to me) for my 73 Cuda. The biggest problem is that you do not have a lot of space back there and if you want to keep it all under the car (verses cutting into the interior, under the back seat) it can be a challenge.
You did not specify what type of 4 link you are after but from what you wrote it sounds like you would be after a triangulated 4 link (which is what I built). I have my top bars going from the center out at about 40 degrees or so and then built a cross member that runs between the frame rails but is flat to the floor and fully welded in. I also reinforced that bar on the inside of the car. This is the bar that my top bars of my triangulated 4 link connect to.
The next issue is the shock mounting, you cannot use the stock mount as it is because it is not intended to take the weight of the car so you have 2 options; cut it out and replace it with something else to mount the shocks to or reinforce the stock one. This is route I took, I cut out the stock shock mount and essentially modified it but remaking the area that the shock bolts to out of 1/16th plate steel, then I made internal gussets to eliminate flex and then I welded it back into the car. I am using Strange coil over shocks in the rear.
Good luck on the project.