* Engine size: (e.g., 4.3L V6, 5.0L V8, 5.7L V8, 6.2L Diesel, etc.) The engine dictates which transmissions were available.
* Body style: (e.g., C1500, C2500, C3500, Silverado, etc.) Heavier-duty trucks typically had heavier-duty transmissions.
* Transmission type: Was it an automatic or a manual transmission?
Once you have this information, you can consult a repair manual specific to your 1991 Chevy truck or use online resources like a parts catalog (like those from GM or a reputable aftermarket parts supplier) using your VIN number. The VIN will definitively identify the original transmission installed.