Perhaps this should be titled "Going after the valves from the bottom." The first step in working underneath (which I didn't do when I was working on the belly pan of the Caravel) is get some additional height. When you're thinking of hiking up some 6,000 lbs, my motto is "definitely over-engineer." I had an old solid core door and scraps of 3/4" plywood filling up my shop, so it was an easy decision. Here's the inside of what became two 10" high ramps and the ramps themselves (the ramp is about 30 degrees):
When I thought more about it and the stress of being on uneven dirt, I decided they also needed a 2x8 under them. As it turned out, they needed to be solidly attached to the 2x8s or they slid backwards (an added benefit of the piece of 2x8 sticking forward was that it trapped the ramp as the first wheel rolled up on it, preventing any sliding). The additional height almost got in the way of the drain outlet, but by putting a scrap of 2x lumber forward of the contraption, I managed to get enough clearance.
I admit to being surprised by how high the forward wheel was off the ground, here:
As it turned out, I didn't need as long a ramp (48") as I thought. I think it could be substantially shorted and still do the trick. I think you only need to get one tire up on the horizontal surface. Maybe 30", or even as short as 24". Shorter would be better--you might even be able to pick it up.
As you can see, clearance was excellent, I could almost sit up underneath. Actually, I wish it was about 5" lower, since I couldn't quite reach up to all the fittings when lying on my back. I think a ramp height of 7-8" would be sufficient if all you were going to do was work on the rear end. If you're doing the whole belly pan, I think the ramps should be as high as possible, considering the clearance of the drain, which is about 11" (give or take, depending on the shape of your axles).
Zep