super simple swap the flywheels are the same balance use the one from your old 302 as well as the block plate and starter so works with the c4 the bronco has, I would try and use the nice serpentine belt stuff the newer 351w came with if posible it allows the use of underdrive pullies and the new alternator and p/s pump which are all good, I am not sure if the pan on the 91 is double or front sump so that might have some clearance issures...should be a cool swap