Regal V6 mounts sound right. The Buick 231, 3.8L V6 and all small block Buicks use the same mounts. I'd say that's a good place to start. The 340 is a nice engine. It was only built for two years, 1966-67. It was superceded by the 350, similar but not quite the same. Heads are different on the 350. The 340 uses the Buick 300 top end and the Buick 350 bottom end. Believe it or not, aluminum big valve heads are soon to be available to fit the 340.
Greg