How hard is it to put a 351 in place of a 302 on a 1991 f150?

it will fall right inYou will have to modify the exhaust to match the manifolds, and if you do not use the electrical harness from your 302, you need to make sure to swap the engine computer. The 352 does not use a pressure sensor on the Power Steering pump and will cause the check engine light to come on. You can splice the connector from the 302 harness into the vehicle wiring, the 351 harness does not have the pins on it's side of the harness.