I'm not all that familiar with the single plane M1 for the 340. What carburetor flange does it have? The single plane M1 for the W2 heads has a 4150 flange and the single plane Magnum M1 flange has the 4150.
If it has the combination speadbore/4150 I would send it to the "High Preist (sic) Executioner" and have him weld it up and shape it to the 4150 style. Though the big block style still works pretty good without the modification.
