There is nothing wrong with a 6 volt system. The main rule is "good grounds". The second rule is "big battery cables". Many people make the mistake of replacing the cables with the 12 volt ones from the FLAPS. (That's the Friendly Local Auto Parts Store) They won't carry the current you need for your 6 V system.
As to the theory about a 12 volt spark- Most 12 volt systems use a ballast resister in the ignition circuit to reduce the voltage to the coil. I thought it was to keep the points from burning up. This is the first I've heard of the theory about "...........and will affect piston and cylinder life, and head and valve life."
Congratulations on your new truck. If it were mine I'd keep it 6 volt.
