with short lengths
of 3/4" galvanised steel pipe as spacers


I agree, and was planning to use large diameter washers at either end of
the spacer pipes. Tack-welding the washers to the pipes is probably a
good idea too.

Pipe/washers might rust, especially if welded. What about slices of heavy
PVC pipe...perhaps held in place by screws inside the circumference and then
furthur clamping by the bolted on "rail"? You might even be able to find
some kind of coupler or fitting that would be the right configuration.