On the old 2 cylinder John Deeres, I have never had a problem using the old 6v starters after changing the system to 12v.
I think I read somewhere a starter does not know what volt it is.
I have used the tractors many years with this conversion, and never had a problem with the 6v starter.
I have never converted a truck, but I don't see any difference.

57 GMC LCF 370
55 John Deere 40-W
59 John Deere 430-W
2000 GMC 4x4
56 John Deere 420-W
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