Ok, so I measured the axle tube this evening and found the following (for future googler's sake):
Distance between U-bolt ends around axle: 3 3/8" with about 1/8" clearance
Diameter as measured using string and circumference: 3.24"
which I think translates to 82mm tube if it's all metric. Anyway, 3.25" spring perches would do it if they need to be replaced.
I'll take a look at the front of the perches to see if they are good enough to use. Maybe a picture later too.

Chip