Just an update for anyone who might find this thread later. I only needed about 4" of additional clearance for the crane to fit under the lower chassis. So I took some scrap treated 4X4's and added a short length of 2X4 on top of them which gave me 5" of total rise. I put these under each tire and added a couple of wooden stops to the rear ones (so the chassis can't roll). It serves the purpose even though it's not terribly impressive looking.
I really liked the idea about using the same size material that you use for the risers to slide under the car's low points to check clearance. I wanted something that was as wide as the chassis that I could slide under easily and check clearances. I was out of 4X4's (and they're heavy and hard to move around) but I did have some 3/4" PVC and fittings! I took a piece about 4' long, put 90s on each end added about 2" of pipe to the 90s and then put a "T" on the end of that for a base. Once I adjusted the short "legs" to the correct height, I marked their location, took 'em apart and glued 'em back together so they won't move.
Cheap, lightweight, effective.