Back in the day I remember seeing a couple of r16 bell housings that had been modified by adding a bulge for the crownwheel on the other side and modifying the clutch release arm to work in a similar way to how a 12G one did (similar to r17 but on opposite side). Then you use a 16 input shaft and bell housing with your r12 box. It’s a lot of stuffing round, and the ratios in a r12 box are all wrong for an 807 anyway. Also. People talk about the r18 block having the same mounting castings as r12/15/17, but that’s not my recollection (I may be wrong but remember stuffing round with one to get it done and eventually just modifying an 807 to take the 18 sleves).