WIW 66 Barracuda /6 3 on the tree a/c 20K miles

Thinking about buying this car.
Never owned a slant before.
Not crazy about 3 on the tree.
If this really is a 20,000 mile car...
How much value would I kill by swapping in 4 on the floor?

I confess, I may be looking at a flip.
If it is a 20K survivor, I might be able to make a good profit and fund other projects.
I would also be OK driving it for a while, and it wouldn't bother me to keep it (with the 4 speed or o/d)(...and maybe a turbo:)).

Have not seen the inside.
If it's factory a/c (not dealer). how odd would that be??

Assuming no body work...What's It Worth?