I put a '49 mercury body on a '69 Buick Skylark Chassis, floor pan, and firewall. My "system" was to lift the body off of the Merc chassis with a series of all-thread rods on all for corners. I then rolled the Buick chassis and floor pan assembly under it. I could lower the Merc body down over it keeping every thing level and trimming the 2 as I went so they mated. Brian is right. It is a big job but really worked well. The car drove like a dream. I personally like to butt weld. There are lots of people who disagree with me but it is my preferred method. With a butt weld there are no hidden areas to catch water. When I was done I unbolted the assembly from the Buick chassis. Picked it back up and WALLA, there it was swinging from the shop ceiling waiting to have the bottom painted and reattached to the painted Skylark chassis.
Good luck to you. I know I would like to see pictures as you go. Surely others would too.