I don't believe that the Mach speed of the plane had anything to do with the engine selection. The technical concerns are more with pod and pylon aerodynamics, systems compatability and integration, weight of the installation, what to do about the center engine inlet and exhaust and cost vs. return on investment.
Since the conversion was never done, it can be surmised that it just wasn't practical. Two approaches were and they are seen today as the Valsan (JT-8D
-217) and QF
TAY) conversions. Both were undertaken to take advantage of existing engines that provided improvement in noise and exhaust emissions as well as thrust vs. fuel consumption.
was engineered for UPS and provided the greatest improvement. However, improved performance in fuel consumption and noise was offset by poor thrust availability at high altitude. The plane perfomed well for takeoff and low altitude climb and cruise but poor, as compared to an unmodified plane, at higher altitudes.
Only the beancounters can say if the high cost of engineering and certification paid off for UPS. I don't believe that any of these remain in service. FedEx still operates the Valsan conversions as well as a large number of 727s with their original engines albeit with newer noise supressors in the exhaust pipes.