A 6v system will turn a 6v starter just fine as long as you have the following:
1. A good battery (Optima makes the strongest 6 battery)
2. A good 6v starter
3. A good spark (a 6v Perronix system is great for this but not necessary).
Most important:
1. A proper gauge positive battery lead (from the battery to the starter). Most battery leads from the last 30-40 years are 4 gauge (fine for 12v). The minimum gauge you need is 2 gauge but 00 (2/0) is the best for a 6v system.
2. A proper gauge negative battery lead (from the battery to the starter). Most battery leads from the last 30-40 years are 4 gauge (fine for 12v). The minimum gauge you need is 2 gauge but 00 (2/0) is the best for a 6v system. I have not been able to find a braided ground lead that is better than 4 gauge (12v) and now use covered 00 (2/00) leads for all my 6v ground leads.
One more thing to consider. Chevrolet trucks ran the ground from the battery to the frame behind the battery. And, then there is a ground lead from the frame to the clutch housing (under the foot-start mechanism). I have taken a lead (sorry about the pun) from 6v GMC trucks and run a long ground-lead cable from the battery to a starter mounting bolt.
Then, also make sure you have good ground straps from the engine to the frame (I do it on both sides of te engine and frame). And, if you have a radio, run a ground strap from the head (behind the distributor) to the firewall.
Or, you could convert to 12v.
