> Is it possible to put a Unibus board in a qbus system, I know it > won't work, but will it physically fit into the backplane?

Yes. Unibus and Qbus boards (and Omnibus boards for that matter) all use the same connectors -- 36 pin (18 pins to a side) 0.125" pitch edge connectors. A Quad-height board (4 connectors) for any of those buses is the same physical size. Putting a board in the 'wrong' system normally causes other things not to work, and may even cause damage, though.