I would and have just run the original starter on 12V, never had a problem and drove my truck every day for a few years.

If you crank it for minutes at a time the 6v starter on 12V will be an issue, but if you have to crank it that long you already have issues...
Keep it simple and use what you already have, it's not that hard on the starter for a few seconds at a time and if it dies they're cheap (used) and real easy to replace.

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-