Billy,
It's difficult to pick the right one without dealing with rubber every day (I don't either) I've found the little comparison scale McMaster has in the catalog very helpfull before. The one where they show erasers, shoe soles, bowling balls and such as comparable to what durometer numbers.
http://www.mcmaster.com/#about-firmness-ratings-for-rubber/=g8tfpm

I'd guess for cab mounts you want something in the "Med. Hard" range.

Mark, curious to know more about how you machine rubber, what sort of tools? I've tried it on occasion with my manual machines and can usually come up with what I need in the end but it never works out how I expect, it's tricky... the soft stuff never works for me unless frozen or simply cut/sheared/punched and not machined.


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-