Just use the setup from the 6. The flywheel, starter, & bellhousing will fit either engine. As long as the 350 is a 2-piece-seal version, and there's no problem adapting the clutch cross-shaft pivot ball on the driver's side, you should have no problems.
Look at the lifter covers on the passenger side of the 6. If they're ~4" tall, it's a 250 (assuming the engine is original). If they're more like 6" tall, it's a 292.