Nice work on the original saddles.
I think with a sturdy paper 3-3/8" diameter template you can layout your cut line and use the grinder and sand paper trick on your new spring perches. Lay it out so equal amounts are removed front and back of the arch and zero at the top.

I don't think you gain anything by starting with what I've seen of the commercially available spacers.
If they need modification, welding, or machine work for the pilot after the fact may as well start out and have made exactly what you want from one place the first time. If you purchase spacers be sure the ID is small enough (close to 3-3/8") so there is material to turn a pilot from.
When I was considering these 1.25" spacers I was either going to band saw them out of some stock I have on hand and bore the inside out on the lathe. Or have them plasma cut to size outside and leave a little extra material inside. The steel supplier does plasma cutting quite reasonably, and free delivery. In both cases finishing machine work on the lathe and mill.

Welding the thick spacer to the backing plate is one idea, but I personally try to avoid modifying parts that don't really need to be. Later if you need another backing plate it's an easy junkyard find and install if you didn't modify first one.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-