Before blaming the starter for "slow cranking", go over all connections in the battery/starter circuits. Clean the battery terminals, clean and re-mount the ground, and make certain the starter itself is mounted to the block tightly and the face of the starter and the block receptacle are clean. The starter grounds to the block, the block to the frame/body, and the frame/body grounds right back to the battery negative terminal.
If the starter spins the engine freely when the distributor is slightly retarded, then that indicates an ignition timing issue. It's possible that the mechanical advance unit inside the distributor is stuck/binding in the "advanced" position. After removing the cap, turn the rotor by hand and it should "spring back" freely through an arc of approximately 20 degrees.
There are several variations of that common delco starter used in IH apps, the difference is in the length of the "nose" and is very slight. Some IH apps use a steel spacer plate between the starter housing and the engine block to space the starter nose back away from the flywheel which prevents starter drive/ring gear interference. Other IH apps do not use the spacer. When obtaining a reman starter for this stuff, ya gotta compare the nose of the units closely in order to make sure ya get one with the correct dimension.
Many times, when these starter cores are remanned, they get a mis-match of correct parts thrown together so that ya have to go through several of 'em to end up with one that is correct. And...it's certainly not unusual that a "reman" starter is crap right out of the box.
If you know someone who has an inductive (clamp-on) dc ammeter, then performing a starter draw (current) test will tell ya if the starter itself is bogus. Typical current draw on one of these motors is between 85 and 150 amps when cold and cranking over for no more than ten seconds. Any more than that and either the engine is real tight, or the starter itself is crap. These engines in stock form have a very low starter torque requirement comparatively speaking.