If your jumper cables are too small they should just get hot. They can get REALLY HOT, so be careful. The lack of any reaction from the starter motor seems to indicate a discontinuity. Does a foot switch starter spin when the ignition switch is off?
Assuming that it does, you should be able to hook the negative jumper to either starter mounting (to bellhousing) bolt and the positive jumper to the normal terminal, and the starter should bump with the footswitch pushed in. This is for Chevy's only. Early GMCs used positive ground so you would have to reverse the jumper cables.
Even with the cables hooked up backwards the starter motor should spin backwards, but that isn't good for it.
Check for resistance between the place you want to hook the negative terminal and the positive battery connection terminal with the footswitch pushed. It should be close to zero Ohms. If it iesn't, try the inner terminal. If you still don't have a low resistance, there is an open, or some other kind of high resistance in the starter motor. It might be corrosion or arc marks between the brushes and the armature.
