340camshaft with stock conveter?

More idle advance is the standard answer, but at 20 inches of idle vacuum,You are probably running enough. Still 20* is a good target.And for sure do not hook the Vcan to manifold vacuum. Check the T-port sync. You may have to close the primaries some and crack the secondaries instead.Jus a wee bit atta time, till it works.
Don't forget to limit your power timing.
And most importantly Be sure the power valve isn't dumping, as the vacuum drops.
That cam with proper tuning should idle just fine in gear at 650 to 700. Or should I say 700 to 650. With a lil luck and a loose engine I think even 600 might be doable
Typically loc-up TCs are a lil looser anyway, cuz, you know, they lock up at hiway speed, so they can be looser, down low.See what I did there? I legalized a run-on sentence with,um,lotsa,um,commas,lol.