It depends on several things, how many miles on the 366, how you want to use the truck if you swap motors, are either of the engines burning oil/smoking? The dieseling can be addressed and probably has nothing to do with the valve train. The 366 is a true truck engine, low RPM torque and heavy duty castings, the 454 is a good motor but not the same animal. The 366 probably has a Holley carb on it with an anti-diesel solenoid and a governor, I can't say if the 454 has a governor but probably a Q-jet carb, both carbs can be rebuilt if they have not been tuned with a hammer. Just a gut feeling here-I would keep the 366 in it.
