When having custom rods made, the whole width of the rod will stay the stock GMC dimension, whether you make the journal size a BBC or SBC or some other size. The BBC journal(IMO), would be a better choice to offset the journal with since it is closer to the rod width of the GMC. Some of the other Hudson and older production rods that are longer than the GMC are a good compromise to cost, but quality race bearings aren't going to be available for those rods, and thats why the BBC and SBC rod journal sizes are more common to use in race applications, if thats your intention.
