I guess the old hobbs meter oil pressure activated switch used to be smaller in diameter. The ones that I could find from Vans and on Aircraft spruce do not have enough clearance to be installed into the suggested location from Vans on the firewall. I was going to go with the showplanes engine mount switch bracket, but that adds another hose under your firewall. I ended up creating a couple of spacers for the standard rail. I was going to just fabricate an entirely new rail, but the only 6061 I had on the shelf that would have worked for this task was 6" x 1" which just seemed like a lot of work.
I made two 1/4" spacers which I will add some lightening slots into at some point when I find out how much I need to space the rail out.