First of all, scratch the jumper cables and use a pair of good-condition battery cables, 18" long or so. You just won't believe the amount of voltage drop in 10 feet of jumper cable!
Havr you tried to turn the engine by hand with the crankshaft center bolt, with the spark plugs removed? If it's stiff that way, it's not reasonable to expect the starter to wing it over. Take the plugs out, squirt some oil into the cylinders, and turn it by hand until any tight spots work out. It sounds like you've got a couple of rusty cylinders that are stalling the starter down. One that's been stored that long without a lot of special attention to make it airtight is almost bound to have some rust buildup in thre cylinders due to open valves and humidity getting in and out of a couple of cylinders. I always seal off all the intake and exhaust passages, and fill the cylinders with oil before any long-term storage.
Jerry
