Okay.... I saw this post and figured that SOMEONE would chime in....
The truck engines had a LESS restrictive intake tract. THATS the reason that the dealers will likely tell you. I can't see 20 HP there though. I think that there is more to it than that. TRUCKS have a different set of emission standards than cars in many cases, and its quite possible that the additional HP could be the result of the preset tune in the computer. The cam and heads are likely the same.