I second Bartimos' comment about rolling the chassis out from under.
When I had to replace the frame on my 1963 LWB, I rolled it into the garage, jacked the truck up, built an A-Frame through the windows with the doors still on, then unbolted the Cab from the frame. After it was free and clear, I pulled the tires/rims, installed a set of bare rims, no tires, and set it back on the ground. Then rolled it out on the rims, leaving the cab suspended on the A-Frame. This made it easy to swap the motor/trans as a unit into the "new" frame and swap the rear end as well. Then it was just roll the frame back under the cab, jack it up, bolt it down and install the tires/rims again.
I did use a couple of straps to the rafters to stabilize the cab while it was on the A-Frame, but that was more of a precaution than a necessity.
