So I snagged a pair of X heads...

Those X heads are considered to be the best factory casting that was made on any produstion 340. Thats a great score. The J heads are ok too, but were prone to cracking between the valves when overheated. I used to work in a machine shop, so yes I have seen a lot of these come through the shop, over the years. 400 H.P. is pretty easy to obtain with these heads.