OK you're in luck, the '91 has the all mechanical AOD trans so installing the 5.8 is as simple as buying a 28oz AOD flexplate for it, you cannot under any circumstances use the E4OD flexplate that is currently on the 5.8, the front-back offset is different and using it will immediately destroy the AOD transmission. With that done you then need to source a 5.8 PCM for a truck with a C6 auto(3-speed) or manual transmission, you cannot use the 5.8 PCM in the '90 because it will throw a mountain of codes without that E4OD trans connected, and moving that trans into the '91 is a big job since it will involve a lot of wiring.