Nothing cheap about putting a turbo on a 4 cylinder!!!! For the same or less money with the combos Mike suggested you can swap to a V-8 quite easily and cost efficiently. Switching to a 6, be it an I-6 or a V-6 is probably going to be just as expensive or possibly even more then a V-8 swap. Resale considered, a V-8 Mustang of that vintage will attract more buyers then a weezy old 6 popper.....

Lots of V-8 conversion parts around cheap, check eBay and other resale sources.... If you're going to go to the work and expense of switching engines, might as well go for the gusto!!!! Or, as they say on TV....."I could have had a V-8!!!!!"