If you go with a 6.2, 6.5, or 6.6 Duramax, it should be able to be done without too many issues. The real trick is going to be getting the correct bellhousing, as I don't think that the Deisels use the same pattern as the SBC. Trans shouldn't be an issue.
1 question, though. How big of a goosneck are you wanting to tow? To be perfectly honest, if you're wanting to tow a gooseneck, you really should consider at least a 3/4 ton truck, or better yet, a 1 ton w/dual wheels. 1/2 tons don't hold up very well under the streeses of a gooseneck trailer.
