Rick,
If you want to get rid of the 55 Chevy axle, here is a lift axle you could use. ART-Neway 505-555 Pusher Axle, Drop Center Might be a little to modern, though.
Here's a steerable ony: Watson-Chalin Steerable Lift Axle
I know Plaskit makes one, too; but I can't seem to find a picture of it. Chris at Sourkraut['s Model Trucks should be able to help you out. He's a great guy, and a wealth of information. If I were going to keep this tractor as a "heavy hauler", I'd keep the lift axle. If you're going to make a highway tractor, not really needed.
How about a Mercury sleeper: Mercury Sleeper
You might be able to find something you need from Dave, too: American Industrial Truck Models