One other thing, not yet addressed here. Pull the starter down, and see if you can get new bushings for the commutator end plate (or a new comm.end plate). and also the drive end bushing. Those bushings, when worn, can let the armature far enough off center, that it can't develop rated torque. Also, check if there is a ground strap, connecting the engine to the frame, and if it's making good contacts.
And, while the starter is apart, make sure the brushes are in good shape, and that the commutator is clean and bright.
If the battery would be grounded directly to the engine block, it would actually be better, because it would eliminate any battery to frame to engine problems.
My '70 convertible has been in storage for around 30 years, and when I parked it, the starter barely pulled the engine over to get it going. The first thing that will be needed for it will be to get the starter off, and do to it, just what I said above. It has tired out bushings.