If you completely remove the two rails, my thoughts are to replace them with curved tubing starting at the lower neck, attached to the frame downtubes at about the same height as before, and then proceed somewhat arched and close to the rear of the top frame-tube with appropriate braces. The idea is to strengthen the neck and provide room to remove the valve-cover and head while the motor is in the frame. Take a look at the CB750A, Tracy, and Rickman frames. Each offers good ideas. I think that Rickman (and/or Tracy) lowered the frame by extending the front downtubes (cradle) about an inch and repositioned the rear mounts. While your at it, think about strengthening the swing-arm mounts and/or raising them a very small amount to accommodate a slightly longer and wider swing-arm.