At this point, what I've got is fairly simple, almost too simple. Obviously it needs some polishing, but I truly appreciate the simplicity in mounting a pulled on a shaft I can mount anywhere. I plan on using this pulley, mounted on a 3/8" shaft.
The part I'm working on is the mount for the shaft. Ideally, the pulley is mounted right above the vertical members of the cart. 1/4" sheet metal sound good for the mount. If my math is right, the pulley has to clear the top of the member by 1 7/16" plus a little extra.
It's a fairly simple part, but it serves its purpose. The shaft with the pulley will rest right in the top. The shaft can be welded to this piece with relative ease. Most importantly however, is that I can place these or very similar mounts nearly anywhere I needed them.
This is how I envision the entire assembly to look.
Again, fairly simple, but I don't foresee any major issues with it. Having this done definitely relieves some stress, and I can focus more on the tragedy of a cart I have planned out.