Or, does OEM mean that the heads from a Ford can’t be used on a Chevy or Dodge etc. How that would work with dissimilar bolt patterns and sizes would require a lot of effort. If they intended to not allow aftermarket heads on the engines why would they have size requirements? Original equipment heads would be the same size as original equipment heads. It could simply say “no aftermarket heads permitted”, in the same way aftermarket blocks are prohibited.
3a. Maximum engine size will be 460 cubic inch. Engines must come from factory in one ton or smaller Diesel pickup truck. Engine must remain in stock location as intended by manufacturer.
3b. No aftermarket blocks permitted.