While grounding battery to frame with a heavy jumper to the engine block or bellhousing might be technically correct, it adds a couple of connection points for corrosion, road grime, salt and other contaminants to accumulate in the path from battery to starter and back. I prefer to have a HEAVY ground cable attached from the battery to the block as close to the starter as possible to minimize the choke points for the starter current. Another ground strap from the block to the frame can be attached just about anywhere for carrying light and other lower-current circuits. Ignition, lights, and other accessory circuits aren't quite as critical as getting a good, long-lasting starter ground, especially on a 6V system.
Jerry
