This is the A-frame I built to lift the cab, engine and transmission of my Unimog, for a transmission exchange.
The legs of the frame were made with 16-foot 4x4 pressure-treated wood. I used lawn-mower wheels to be able to move the frame around. Unfortunately, these wheels were not made for very much weight, and flattened out when under load. Solid wheels would be better. I made cross-braces to the other legs with chain-link fence pipe. The wheels and the cross-braces are held by the same 1/2-inch bolt.
The beam at the top was made from a steel I-beam. The legs were cut to fit into the I-beam, and through-bolted.
I also made diagonal braces from chain-link fence pipe, to keep the a-frame from collapsing to the side. These were also bolted on.
The frame had to be assembled laying down. While standing it up, I tied ropes diagonally between the bottoms of the legs, to keep the frame from twisting and breaking one of the legs. I used a block-and-tackle to a large tree to help stand it up.
After using this setup a couple of times, I have found that the wooden 4x4's are not the best material. I have broken a couple of them just in the process of standing the a-frame up. I am now (August 2003) working on the next version of the a-frame, using steel 4x4 pipe. I'll post more photos as the project progresses.