One thing that hasn't been mentioned but may be the problem is the heat generated by the exhaust. I have had this problem in the past, basically the starter gets hot and won't work properly, a heat shield can help with this.
Heat from the block, maybe. The starter is on the opposite side of the engine from the exhaust system - designing a heat shield might be a challenge.
Regarding the 8 volt battery "solution": this might help and might actually be cost effective, if you have a free 8-volt battery laying around. Years ago, this was called a "farmer's cure" for slow cranking, because 8v batteries were readily available in-use on tractors (I dimly recall).
The 8 volt "solution" might also decrease the life of light bulbs and might cause a problem with 6v electronic ignitions (especially if the voltage regulator is adjusted to deliver 8v).
If you do not have an 8v battery, you might as well try the higher probability-of-success solution: get 1/0 or 2/0 battery cables (replace both the power lead and the ground lead). Farmers (and almost everyone else after the '50s) would have used 12v battery cables as replacement cables.
