My book does not list a 3998992 but there is a 3998991 and 3998993 casting numbers assigned. NEITHER are 327 heads, both are listed for 68-79 307 or 350 engines, the 91 being a 74.5cc head and the 93 being a 75.5cc. These were used in really low HP motors.

There is also a 3998997 which is a 71-73 350 or 400 ci engine with a 76cc chamber.
Like Pat says, the high cc chambers make for a low compression ratio.
mike in tucson