Just a post-race update now that the magnetic numbers experiment was run at Lemons / Gingerman this past weekend and we can share our results.
Tech did not care as long as the size, and contrast were correct.
We went out on Friday test day at speed and pretty soon they made an announcement on the PA for whoever was running #42 to come up to the race control to get our number - it had flown off and got run over by another car.
So I cleaned it up and got some matching white duct tape and taped the edges and it stayed on all weekend. Then at the end of the race I removed the duct tape and magnetic panel and it cleaned up nicely and did not hurt our nice permanent (historically accurate) vinyl numbers underneath. My permanent number is an all in one 24-inch 3M vinyl circle with the numbers permanently / professionally printed on it.
After this, I will be putting historically accurate numbers (ford-versus-ferarri) on both my cars and using magnetic numbers if I can't get them at registration.
https://freeimage.host/i/J6zglVf
Separately on the hood (you can see the picture) I had to use a different approach because Mustang (And Ferarri 330P3) hoods are aluminum. I purchased some temporary white vinyl for my vinyl cutter and then covered the underlying permanent numbers with the vinyl and put pre-cut black vinyl numbers on top of the white vinyl.
This also stayed on the whole race, looked better, and was easier to take off - so I think that for the next race I will go the vinyl route. Urls for the vinyl and numbers below:
https://amzn.to/3QGNDJe
https://amzn.to/4brSfeq
Thanks all for the suggestions and support. See you all in Chicago.