Draw out the difference in diameters with a compas on a piece of paper, it's not much difference and well within what you can do with a file or grinder to a scribe line, quite accurately. No milling machine necessary.
The perches wrap around the tube well less than 1/2 way. Even at half way around your 1/8" difference in diameter is 1/16" on either side front and back. No need to take any material off the center of spring perch, so adjust radius tapering from less than 1/16" front and back to nothing at the center. You can cut out a piece of card stock same as axle tube diameter or setup compas and scribe or trace or swing the desired radius on store bought perches. Lay it out so same amount is removed front and back of the arches and alignment is nothing to worry about.
I think you'll find that easier than making perches from scratch yourself, and cheaper than having something special made to your drawing.
Trust in your ability or ability to learn and give it a try.
NorCal, listen to Grigg. You are overthinking this. Grind, dremmel, sand, whatever to get perch to fit nice on the tube diameter. We are only talking 1/16 radius difference. So take off a 1/16 off of the perch radius. Do this WITHOUT bed installed. It's much easier to mock it up. It's all 85% self locating. Forget all that "loaded" stuff. Has nothing to do with it. Make the off-the-shelf perches fit the tube while spring pin is in hole, axle rotated to achieve pinion angle, axle centered on frame and wheel bases within 1/8 of each other. Tack them on while springs are on them. Use the weight of the frame/springs or.... clamps, magnets, hose clamps, vice grips...whatever will hold it all while tacking.
Make sure you grind to good clean metal all around the tube area and clean the perches
before you tack. Make sure you have a clean metal place near both perches for the ground clamp. Always make sure the jaw of the ground clamp with the cable attached, is on the best ground spot.
I would then sit down and stare at it and think about it for awhile. Measure all one more time. Go back and forth from one perch to the other so as not to heat the tube and melt a seal or heat damage a bearing. The tranny tail and the pinion yoke ARE to be parallel and 3-4 deg. If it was possible to hook up the drive shaft, that would be a real good check of X, Y and Z axes and pinion angle before final weld.
When you are done, touch up the paint and move on. No shims.
P.S. What does this mean? "the centering hole won't be exactly centered in relation to the
axle cut-outs..."
"Lowering the rear"? You are on your own there.I apologize for too much detail if you are experienced already. It may be useful for other users "down the road".
