Yes, a BBC will bolt to the SM420. The later bell housings that were made for the SM465 have the same bolt patter, but the center hole is bigger, so it does fit the SM420.
The V6 is a torque monster. Problem is that they don't like to be run wide open for extended periods. They are also expensive to repair, parts are getting hard to find. If you go this route, get the 351, it's a far better engine than the 305. It will pull the hills, but won't do it as fast as the BBC.
I remember dad driving up Wolf Creek Pass in out 64. 230 and 3 speed, with a topper and pulling a tear-drop trailer. Went up the hill at a solid 50 mph in second gear. If he had the 292 he probably could have done the same pull in 3rd gear.
The 292 will get down and grunt. Wish I could get my hands on one for under $100, they're not an easy find here, farmers used them on irrigation wells.
