You’ll basically need to weld in anchor points for the belts. Not a ton of fun.
As to the value thing… well, if you have a car with matching numbers and a VERY clean rebuild (all factory parts, factory paint and no rust repairs, then yeah, the rear belts are gonna hurt the value of such a car. But how would you get ahold of such an astoundingly valuable car in the first place? if you think your car’s worth about 15-20 thousand, then it’s not a frame-up factory parts rebuild. If it IS such a rebuild, you’re driving a 50 thousand dollar car. ***???
There are after-market companies that sell anchor points and even three place anchor points for classic ‘Stangs. A three point belt is best as there is a body of evidence that suggest (SUGGESTS… not PROVES) that rear seat lap belts can be worse than no belts at all. Three point rear seat belts are however widely understood to be better than either of those options.