Hy bugnut, welcome, if all of the stomp start linkage between the firewall and the starter has been disconnected there should be no reason once the two starter retaining bolts/nuts are removed that the starter should not come all the way out, dirt buildup in the bellhousing may be holding it, wiggle the outer end of the starter and it should come out. Slow cranking could be caused by many things, you have already eliminated some by cleaning all your wiring connections. It has been a long long time since six volt electrical systems were used on vehicles, todays battery cables are too small in diameter to flow twice as many amps which the six volt systems need to do their job efficiently, so if your cables do not look larger in diameter than your daily drivers cables then you need to get proper six volt size cables or double up on the ones you have. As I said slow cranking can be caused by many things, but I would start with the battery cables and see if that improves the situation, hope that helps.