I'm a 70 roadrunner owner but the 70 GTX has always been on my list of mopars to get one day. The market for them seems more premium than RR I'm guessing because of lower production numbers. Say you had a rough but restorable 70 GTX. It was originally 4bbl 440/727 by fender tag but you wanted a four speed. Would it hurt the value, enhance the value, or no change to convert to 4 spd? Or just keep hunting for a factory 4 spd?
You asked specifically about affecting the
value of the car to convert it to a 4 speed.
You did
not specifically ask about whether you could/should do so regardless of value....
So, to answer your specific question - anytime you significantly alter an original "pedigree" car such
as a 1970 GTX, it
does detract from the market value of the car.
The question of whether it's worth it to you to do so is an entirely different one - if you don't care
to devalue the car a certain amount in order to satisfy your own desires as some of these guys suggest,
by all means please do.
Just know that it will detract from the cars' market value to do so
if it was original to begin with.
Now...
A basket case, hacked up car, one that no longer has any original drivetrain in it, well....
saving a car that was not roadworthy prior to the conversion, you'd actually be
helping value, eh?