The first thing that comes to mind is that you need even development. The 16mm motion picture processors (of the tabletop type, manual, like the one on the web site but with two drums) allow you to crank the film between two spools, to get even development.

If you NEED such long film, do it. Try B&W with one of those processors. However, I'd say that having it processed by a motion picture house is the best bet, as long as you recognize that it will NOT be up to normal photographic archival standards, especially if you use motion picture film stock.

Maybe look into whether any of the roller-transport processors will do it (the minilab machines) or whether the film could be wound around a metal rack in a dip & dunk machine.