MP DISTRIBUTOR TIMING CURVE 340

OP said he can't get the idle any slower without shutting off the transfers, Hello!

Op, pay attention to Mattax; Shutting off the transfers will only make big trouble.
If you have proved that the only air that the engine is getting, is coming in past the primary throttle blades and that the PCV system is properly calibrated, THEN.................Just retard the stinking idle-timing already; There is no other way to slow the engine down!
Then fix the timing with a two-step curve. Ram it in hard and fast to 2500/stall, then slow it down to finish up without detonation.

The 340 is likely to want it's own timing curve, I wouldn't waste any time trying to find a one curve fits all deal.