I agree, grafting in a subframe from a '70-'77 T/A would be a good thing. Not trivial. Be sure to have a qualified frame shop line it up before welding it in. We did that to a '66 GTO a few years ago. Worked well. The WS-6 steering and brakes made that GTO act like a 2nd gen T/A.
Master Power makes a conversion "kit" for the A-body, to use the Delco-Morraine system. It's complete, and very good. About $900. Probably a bit less money in the long run. Ames Performance offers the WS-6 steering box in an A-body flavor, too. So I suppose, 6 of one, 1/2 dozen of the other...