It occured to me today while I was thinking about drive belts that a set of belts behind an engine could be used as a PSRU driving a shaft that then runs over the top of the engine between the valley on top of a V engine and to the front. It would use crankshaft bearings in custom mounts that have integrated tensioner bolts for the belts. There are several massive advantages to this setup. A) Allows gearing without PSRU B) Allows auto engines to be used in standard configuration with accessories on front and drive on rear. Greatly reducing the level of modification required. C) Allows use of off the shelf parts that can be inspected and replaced easily D) Allows keeping radiator on front with fan, greatly simplifying the cooling system and reducing overall length and complexity. E) Allows a higher prop mounting for better clearance/larger prop, much like a Spitfire or Mustang F) AND MOST IMPORTANTLY: Allows gearing that can be inspected and modified easily without a casing around it. Im sure there are more, but this is just what I can think of off the top of my head.