Since my Wista is double extension only, the longest lens I can use in practice is a 270mm (a G-Claron in my case). A top-hat board would help a bit, but that sort of extension only really helps you get closer than infinity for focus. If you cannot really get infinity focus on the base camera your options are limited.
The MPP VII will accommodate a 400mm lens, so this is my camera of choice for long lenses and closer focusing. On the other hand, the MPP does not give much flexibility at 90mm or shorter, while the Wista will handle a fast 90mm with decent movements.
If you have a 210mm or a 250mm as your 'long', then look at the possibilities of enlargement. Or use a camera with interchangeable bellows.