Hi Stumpy,
I found that Signal's pdf file was a swine to open but WTF, I ain't computer-smart.
The 20mm x 52mm axle slots are good, as is the V-notch that locates the tensioner's adjusting screw.
Although Signal's 6mm plate thickness is OK for a sidecar wheel, I'd use the stock axle plate's 16mm (5/8") thickness for a hardtail.
(I just trudged out to the garage and measured my spare swingarm and yes! the axle plate IS 5/8" thick!)
Then the stock chain tensioners will fit without needing spacer washers or being squashed out of shape if you don't use any.
Going back to post #2. Your axle plates' external shape can't be a straight strip as shown on Signal's pdf.
It has to match the angles and placement of your scratchbuilt hardtail's tube structure and that's up to you.
Note that the "magic distance", the space between the axle plates' inner faces after all the welds have cooled, is 8-1/2".
All this is to fit a stock XS650 rear wheel and spacers.
And one last thing; be sure to weld up your hardtail so that your transmission and rear wheel sprockets are in line.
Bicycle deraillieurs run an skewed chains OK. #530 motorcycle chains, not so much.