When to get cheaper aftermarket parts and when to get OEM parts.
OEM parts have the benefit of extensive R&D and testing. Truck has to pass many tests to check for reliability before being sold. Therefore, OEM usually works the best. Aftermarket parts don't have that kind of testing, and many cut corners. However, in some instance aftermarket part may be the same or better than OEM. Especially when people discover a weakness in OEM part, and aftermarket part corrects for it. Other times, aftermarket parts will fail much sooner than OEM parts.
Head gasket: Head gasket is carefully engineered, and often fail if not manufactured using proper material or tolerance. You can tell by all the intricate copper overlay and special texture. Using OEM part here is a good idea.
Exhaust gasket: Not as critical as head gasket, but it does take high temperature, and do have some copper overlay as well as special coating.OEM is probably a good idea here too. If exhaust gasket leak, you will have incorrect O2 sensor reading, and cause incorrect fuel mixture.
Intake gasket, water pump etc: Most of these are just a simple piece of gasket. OEM or aftermarket will be fine.
Rocker arms: Here, OEM is better than aftermarket. This piece is under high stress especially during high rpm operation. Safe to use OEM which has the right material. Newsgroup experiance indicate aftermarket does not last like OEM. In fact, most OEM rocker can be reused.
Pressure Plate: If you use an aftermarket, the clutch feel will be different.Aftermarket many times are cheaply build. Use OEM if you intend to keep the truck for a long time.
Clutch Disk: From news group, OEM last the longest. Unless is a a high quality aftermarket brand, you may be getting inferior material.
Birdfield: OEM birdfield is quite strong. If you are not going over 31" tires, use OEM birdfield. Some aftermarket birdfield like the Longs simply anneld the material so it deforms rather than crack. Makes them actually weaker. Exception are the new cromolly ones. Those actually use better material than OEM.
Roller bearings of all kinds are made by only a few manufactures in the world, and OEM use these same manufactures. Therefore, OEM and quality aftermarket bearings will perform about the same.
Brakes are so easy to replace, no point going with OEM. Most aftermarket brakes are pretty good quality, and you can get fancy ceramic brakes.
Simlar to brakes, this is easy to replace. A quality radiator will be fine.