The majority of the old overdrives are now obsolete. If you are lucky enough to find parts they are often expensive. I know some commonly needed parts are not available for many of not all of the old Clark OD transmissions, a friend of mine was recently running one and had several more for spare parts, still could not find enough good stuff to keep it in service, and he is an excellent mechanic and has good luck finding stuff if it's out there.
If you find a good one that fits it's probably worth a try, but may not be a long term OD solution.
Some of the big ones are really quite big. The 5 speed that came with the V12 I owned for a while could have been ordered as an OD, probably weighed about 300 lb.
Grigg
