I had the sMe problem. What worked for me was a smaller wheel and I put a shim under the front seat bolt. This tilted the seat back a little and gave me more room.
You asked for value. Having the original auto trans adds value. I would the 4 speed over auto any day. But the original equipment in a high dollar car makes a big difference.
I was in for the challenger until you changed it from original auto to 4 speed .
I would keep the Cuda.
By the way I own a 70 challenger 440/4 speed and also have owned a 73 Cuda 340/4 speed.
I was thinking of going TKX but some people say no cutting and it's good and others have issues with fit or getting the right angle for trans not to vibrat.
I had 3.23 and went up to 3.55 and the city driving was so much better. You do notice it revs a little more on the highway, but not by much.
I prefer the 3.55 for better all around driving.
I had the same problem 18v. Bought a new vr and still at 16v. Got my hands on a original vr, 12v. My problem was that even the new vr's were not any good.