Sun is exactly correct! I built a rack for my body in a weekend. Ben also had some plans posted at one time for the one he built. It also makes it easier to detail the underside of the body if it's on a dolly.
As far as the weight, it's a guess based on how easily mine was to handle once I got it off the frame, I'm thinking around 500 lbs. I pulled mine with a cherry picker engine hoist with very little trouble.
Good advice on NOT using just the garage rafters to support the weight. If they are typical roof trusses they are designed to carry a load from above not a hanging weight. You could probably still do that IF you added the support columns directly under where the 2X6s cross the rafters. That way the bulk of the load would be supported by the columns. As mentioned a 4X4 should be enough or those adjustable pole jacks would work also.
Also good advice of not working under a suspended anything. Build a dolly, lift the body off, roll the chassis out of the way, roll the dolly under and lower the body on to it. Granted that approach takes up extra floor space but is WAY safer for all who would be around the project.
I'll be doing this someday so lessons learned will help me (and others) in the future. Good luck!