+1 on what Larry wrote.
The Offy and stock are very similar in plenum and runner design. Why not use the 1 barrel manifold for what it was designed for and when you need more juice, break the Offy back out and use it what it was designed.
If it will work in the engine bay, get the larger exhaust manifold from a 292 and get a little better breathing on the exhaust side. The 292s flange is bigger but note, the flange is 3 studs vs the 2 of the 250.
Regards,
Rick
