Bill,

Have you checked for vacum leaks?  One way to this is to turn on a
propane tank and direct the unlit gas to the intake in differnt areas.
If the car idles up it has a leak that is sucking the propane.

I like to see idols go down, but your idle issue may be a vacuum leak,
WTF is the purpose of drilling the carb base?

Are these two barrel carbs? Is so make sure that both butterflys are closed
when the idle screw is all the way out. Sometimes they are out of alignment.

Where (and why) would you drill the carb base?

The main jet has little or nothing to do when the carb is on the idle
circuit.  Changing jets won't effect the idle speed (I think it would
be difficult getting different jets for these carbs anyway).

If you have disconnected the linkage and backed off the idle speed
screws all the way (in fact, removed them) AND the throttle plates have
completely closed because of this (they should) and it still even runs
(let alone at 1200 RPM, then there is a vacuum leak.  Check carb base
gaskets, manifold to block gaskets, throttle shafts, any vacuum
fittings (dist advance, wipers, etc.) for leaks using Michael's
sugestion of an unlit propane torch.

-Dick-

Back in the olden days carb repair guys had a chest of specialized
feeler gages to set the blade angle, pulloff angle any and all such
appatures had a feeler gage.

 Presuming that you do not have a vaccume leak becausu you have tested
it with propane.

You said that you had backed off the linkages and tried to adjust the
idle.  Have you looked down the carbs throats at idle?  Is one of the
carbs drippin internally?  Are the blade angles set the same.  Are they
a matched pair.....ie.  jetted the same? Are the float levels set
identical.

One more thing that I forgot.......what is your engine timed
at........If it is to far advanced it could idle to fast too.