Is it possible I have a crossed wire on the solenoid assist?
That starter solenoid is a real bugger to access, use two nuts on the big post, run the first one down and just snug it, then put your cables on and use two wrenches, one on each nut to pinch the cable tight between, that way your not stressing the post and less like to break it.
On the slave solenoid, you should have a yellow/red wire to the S terminal, and a Purple/blue wire going to the I terminal.
If you do, put the shifter in forward or reverse (Just to engage the neutral safety switch) and turn on the battery switch and see if it continues to crank. Make sure the prop is clear, if it does crank over, the prop will be turning too.