I never did a starter on a 3.0 but have done one on the 4.3 V6 at least 3 different times over the years. You may need to get a thin wall socket to be able to turn those bolts. Be careful and make sure you are turning them the right way since you are using the ratchet wrench upside down lol
Also before cranking the new starter over, you might try taking out the plugs and ground the coil wire try cranking it with the plugs out to make sure that don't have a water in the cyls problem.
Not common but it does happen....that would cause what LOOKS like a starter problem but really is something else.