Ole Bill, Electrical Resistance is your underlying problem. Before you go and do some dramatic "fix" as you propose, consider correcting the problem itself rather than "fixing" the symptom.
Resistance is the problem, and to rid your electrical system of this problem, you begin at the battery, following the paths of electricity through the circuits back to the battery again, cleaning and assuring an unimpeded flow of electrical current. Preventing future resistance at these points is accomplished simply by shielding the contacting surfaces from oxididation by the proper use of a dialectic grease, available at any auto supply. Also, of course, undersize cables and wires must be replaced with their 6v capable counterparts.
The gorilla in the room is the starter motor itself. Worn brushes, weak springs, armature wear, and mica interference being the main causes of resistance in starters, any competent motor repair shop can properly rebuild your starter in a matter of an hour or so. This step alone should rightly be considered part of all routine electrical system checkups, yet is so rarely done that it is likely that half of all 12v conversions have bad starters.