91 is easy, all the wiring behind the dash is the same, so it is plug and play. Buy the 5.0 bird donor car. You will want to use the ECM, under hood wiring, and the harness that goes from the ECM through from the firewall. The k-member is the same up through 92, so the 5.0 bird motor mounts will land in the right spot on the SC I-member and bolt right up. If the SC is an automatic, then keep the SC trans (better OD servo) but use the 5.0 flexplate (different balance from SC to 5.0) and the 5.0 torque converter (slightly higher stall speed). If the SC is a 5-speed, then you will need a clutch, flywheel, and starter for a 5.0 5-speed F150, or you will have to have the SC flywheel rebalanced to the 5.0 50oz imbalance.