Seeing how bad the shaft looks, your problem is probably going to be revealed when you remove the sheave: The crusty shaft has eaten the center of the sheave and it's now oval, meaning it turns when tested with no load but poorly when loaded. Had a similar problem on our outhaul, although different boom end fittings.
The plug will have to come out to remove the shaft, so get to work on it. I doubt it's supposed to be tapered, probably someone already battled with it. Can cut a groove in it and try that, or drill it away and replace plug and shaft (shaft probably needs replacing anyways).