On my '80 CB650 the lower right rail of the frame unbolts. Two bolts through the motor mounts, two bolts roughly behind the brake pedal. It allows removal/installation of the engine with the oil filter in place. I found that laying the engine on its side and lowering the frame over it was pretty effective. I bolted up the left side motor mounts, then stood it up, replaced the rail, and bolted in the right side mounts.
Also, I found it easier if the frame was all alone... no wheels, handlebars, fork, or anything else. I was able to lift the back end to put a wheel on, then used the center stand and a jack under the motor to lift the front end to put the front wheel on.