The reel assembly is actually essentially the same. The main difference is in the reel baskets, and even those aren't that much different. The big difference is where the nub is in the basket where you line up the notch in the reel strips. It's in a different place on the slant top baskets (in order to display the correct symbol in the slant top reel glass, which is positioned differently compared to an upright machine).
In theory you could use a slant top reel basket in an upright machine, but you'd have to add an additional notch to the reel strips to have them align correctly.