On my 37 starter, I had to replace the brushes and turn the commutator. There are 4 brushes in there. I find it suspect that it will turn with no load but when load is applied it just clicks.
Are you setting 6vdc at the contacts and then at the starter? if so then the starter would be in question
If you are not getting 6vdc at the output of the contacts but do get it at the input side, the Starter sol is in question
If you are not getting your 6vdc at the starter but are getting it at the contacts then the cable is in question
Have you tried to bypass the sol and run your 6vdc directly to the starter?