I concur that the SM420 will bolt up, but you'll need the proper bellhousing. However, unless you already have a 420, you'd be better off with the 465, since parts are more available for the later tranny.
If your truck is a 68 (like your name), you may already have bellhousing mounts available on the frame. On my later 77, I had to come up with a different way to mount the 465, since the original bellhousing mounts weren't going to work.
I also went with a hydraulic clutch linkage, to replace the original rods & z-bar deal. It works great, but don't underestimate the amount of work. It took hours of looking just to figure out what to make....
One other thing, the starter used with the 465 is bellhousing mount, not block mount.
Greg
