Leith,
I just ran a tape quickly over the XL350 swingarm that is in my '72-'73 XL250 and compared it to the original swingarm from the 250.
Swingarm pivot to centre of rear axle slot, XL250 = 435mm, XL350 = 460mm (give or take a mm or so, my eyes are going as I get older).
Swingarm pivot to lower rear shock mount XL250 = 385mm, XL350 = 415mm.
Col