Its a modified aftermarket GM part, a "manifild filler neck". I had the bolt holes moved in and the center bored out to match the factory size. Easiest and neatest way to do something similar would be to get a spare tstat housing from a wreckers, cut a hole in it and get a filler neck welded to it. Then have the blank outlet nipple on the factory tstat housing tapped and run a line to the top of the radiator on the as its the highest point, without this you'll end up with an air gap in the top of the radiator. You also need a plug to block the 1'' radiator outlet that runs to the header tank. Also I found that you have to use at least a 21psi radiator cap not a 16psi like the stock setup or it will loose a lot of coolant on shutdown.