I would say there is no appreciable difference... I did a Cummins swap into a '80 cab from a '92 cab on a '83 chassis and the only thing I needed to change was the bulkhead opening and the heater box opening ( that was the difference between the '80 and '83 cab). Other than that I swapped the entire pedal assembly, elec. harness...everything straight bolt in. So a '92 to a 91 cab should be effortless.