Brad,
I took a look at my 46's rear axle and it, too, appears to have a slight space (approx. .020"-.030") between the lower clamp I.D. and the axle tube. There's no evidence of any gasket ever being in there, but maybe it rotted out ?? I've only had my truck a couple of months and haven't gotting into the restoration yet, so I can't give you a definitive answer.
I checked my Parts Manual and the found the parts listed as the "Seat & Cap - Plate, Rear Spring" - Group #5.403 / P.N. 600176 (right side) and 600175 (left side). Looking thru the different parts, I don't find any liner or gasket listed. It just may be that the axle tubes weren't held very closely - certainly not machined - and the clamp held it tightly enough with only partial contact.
I'm attaching a pic of my set-up, which has a set of overload springs mounted above the axle.....
[img]
http://home.rochester.rr.com/mrplates/46RearAxle.jpg[/img]
Probably doesn't help much, but all I can offer for now.
BTW, if you run acroos TWO sets of running boards, I'll be happy to take one set off your hands !!
-Bob