I replaced my starter to battery +pos with a really flexable red covered welding cable, crimped ends, then quickly sweated with solder, then a shrink wrap to seal the ends. The soldering must be quick with a fine hot torch so as not to burn or melt the cables cover too much. Some say not to solder because of "cold flow" but this should not be an issue since the crimp should permanently hold the terminal tightly to the cable, the lead filling voids and giving a full bond metal to metal. (Sorry I'm old school and still use proper solder)
But I still had "sluggish" start, only when engine hot, the starter nose bearing was shot and I fitted a "gear reduction starter" - its lighter, smaller and a cite better.
Most people think big red +pos cable and forget how the Starter picks up -12ve, there may be a number of earths, motor, trans, frame etc etc etc - any of which after conversion could be missing or loose. I put a nice woven earth strap direct from battery negative down to the mounting bolt on the starter.
There are many preps available to coat terminals, but the best I've found is Lanolin Spray used spareingly.
