you will need the engine chassis mounts to accept the motor mounts from the 8 cylinder. you will need exhaust or headers with pipes and mufflers for the exhaust. you will need a new flywheel and pressure plate and clutch disc for an 8cyl if it is a manual if it is an automatic you will need a heavier duty trans that has more clutches and plates in it and heavier torque converter. and again a new flyheel. you will also needa 350 starter motor and a multiple core radiator say a three to four core for proper cooling and new pulleys unless it is a wrecking yard engine and has them and a new alternaoter bracket assembly and a new power steering pump mounting brackets and new power steering hoses and a new cooling fan and you may most likely need a fan spacer. and a new fuel pump and I am willing to bet your truck has a 5/16ths fuel line and you will need to enlarge it to 3/8 ths from the tank forward for more fuel supply.You may also need a oil pan for a 350 pick up oil pans are not really interchangeable from one model to another. If you live in a pollution control state you may have to install catalytic converters to apease the feds.

To put a V8 in there you need a set V8 motor mounts the rest will bolt up that will put your distributor close to the fire wall but it will fit to make it right you need to unbolt the motor mount hangers and move them forward to the next hole do the same with your trans cross member if you have two piece drive shaft it will work but if it is a one piece you may need to fine a long one