Mounting the gearbox with the engine presents add'l structural and space considerations. The gearbox isn't necessary to run the engine. You can just use the clutch housing from a 901/915 gearbox to mount the starter or fabricate a starter housing.
Some pics from Rennsport last year. Modified 6 > 4 cyl. 911 on what looks to be a regular engine stand with brackets added for oil/fuel tank, control panel, etc. The heavier 911 would need more support on the fan end.
Sherwood