Using a IGH in a bike with vertical dropouts requires some method of tensioning the chain. If you're lucky, you can get the right # of teeth on the front chainring and rear cog, and it will work out right, the magic combo. A half link in the chain might be required. There on some online calculators that you can use to determine the right combo. The chainstay length is a variable. Otherwise you'll need to use a tensioner. This would allow you to use 2 chainrings and a derailleur up front, if you wanted more gear range.