Any 6 volt system needs the biggest battery cables available, and clean tight connections everywhere. Don't assume- - - -unbolt and wire brush EVERY connection between the battery and the starter, and do the same thing with all the grounds. Be sure there's either a direct ground cable from the battery to the engine or the bellhousing, or if the battery grounds to the body or frame, be sure there's also a BIG jumper to the engine from there. By big, I mean a #0 wire or bigger- - - -the cable insulation should be as big as your index finger, minimum. One the size of your thumb is better.
If it still doesn't turn right with clean connections and big cables, you've either got electrical problems inside the starter or a tight engine. Can you turn the crankshaft by pulling on the fan with the spark plugs removed?
Jerry
