My dad and I put an 85 dodge body on a 78 dodge running 4x4 chassis (the 78 body was swiss cheese and rust) using the front bucket on a backhoe and lots of straps we lifted the cab/front end off the 85 and put it on the 78. Another thing we did was use an I-beam in our garage to do as you described with an 86 toyota 2wd. (again good body/bad frame swap with good frame/bad body truck) We put straps through the windows and used come a longs to raise the cab and we also put straps and a second come a long under the front wheel wells. Worked great, but an 86 toy cab/front clip weighs about 1/4 of a 67-72 chevy. You may have issues with height (4x4 frame). If you have the engine in the truck I think it would be difficult getting the grill over top of the motor. If you have the garage/storage space, I'd leave the body on the 2wd until the 4wd frame is ready. No point in setting up your cab lift twice if you dont have to.
My suggestion would be to use your beam (as long as its real beefy) to lift under the cab and then use a cherry picker to lift the front of the grill, slide the suspension backwards from under the truck and roll the new chassis in. I consider this to be very dangerous so please be careful and use enough straps/chains/whatever, and don't attempt this by yourself

(someone has to call 911

)
Edit: I don't know what a single post car lift is, the one with one post on the side and 2 prongs that form a V? You could jack the body above the frame high enough then maybe slide the prongs under and lift?