Sorry , but as far as I know there is no such thing as a drop top mach1. Just having an R in the serial number only means that the car has a 428 cj with a shaker. R code converts do exist but are very rare however , and none of them should be mach 1's. I believe that the car should have a 5 as the fourth digit of the vin , this means the car is a mach1 ( I think ). Keep in mind that the car could also be retagged with a different serial number. The ONLY way to tell if the car is real is to run the vin number through Kevin Marti. Everything else can be faked, shock tower bracing , staggered shocks , vin tags, mach 1 goodies. I would stay away from this car. But , if you are serious about the car , run the vin number and see what happens.
70 SCJ R-CODE MACH 1
88 GT