While the SBC is lighter than the stovebolt 6, the mounting style for the SBC puts different stresses on the frame. The engine side mounts will tend to twist the top of the frame inwards and the bottom outwards. A crossmember style mount will tend to counter this. I don't know of any published analysis on the AD frame that would say whether or not it can handle the stresses, but Chassis Engineering Inc, recommends a crossmember style frame mount for any frame wider than 25 inches (link).

regards,
Leon