I just recalled something important from my own conversion project. The chains and sprockets used by derailleur bikes are narrower than those used by cruisers and internal gear hub bikes. I had no problem with this because I purchased a cheapo 22t sprocket for my shimano hub. The sprocket was made from thinner stock and fit my narrow chain without problems although it wobbled on the splines of the hub. I do not know if special sprockets are available to help you with your conversion but I have tried derailleur chain on normal sprockets and it doesn't work. The teeth wedge into the chain and stick.
Also, the rear spacing had to be narrowed. If you have a shimano 4, 7, or 8 you already have standard road spacing, 130mm.
Last, the derailleur interfered with the rear sprocket. It was designed to clear a much LARGER sprocket than the one I had, but only when extended away from the frame, closer to the centerline of the wheel.
As an aside, I would add that if you have an internally geared bike you are not happy with, try screwing around with the rear sprocket first. You may find room for improvement. I favor older equipment (see sign-in name) and back in the '70's bike makers seemed to think 20-30 rpm was ideal cadence. Adding teeth to the rear sprocket really livened the bike up and made it usable, but I still can only use high gear when the wind is at my back and I am on a slight downhill. I would tailor it down further if I could; not being able to barrel down hills at 25MPH on a 30 year old bike is a small sacrifice.