idle too high, makes undriveable

I guess it could possibly be a vacuum leak, but I think not. Vacuum leaks will usually keep an engine from idling at all. I would say it was definitely carb related. I am not a Holley expert (in fact I don't like them at all), but I would say you have one of 4 things going on:
1. Choke fast idle circuit not disengaging when warmed up. Find the fast idle cam and ensure it is completely disengaged when warm. It could be a problem with the fast idle screw, or the choke linkage travel may need be adjusted a bit so that the fast idle circuit will disengage when warm.
2. Choke adjustment. Kina covered in #1. The choke linkage travel must be adjusted so that the choke plate closes when cold (you will, of course have to hit the accelerator linkage one time on a cold engine to set the choke) and opens fully when open. The choke linkage MUST also disengage the fast idle linkage.
3. Mechanical idle screw and/or idle mixture screws set so poorly that the engine is idling at 2K.
4. It is possible that the secondaries are partially open. I had that happen on a Holley several years ago and it resulted in a very high idle. Look down the carb throat to see. It's been SO long since that happened, I don't remember what I did. I seem to remember using some carb spray and fiddling with the linkage.
Now don't take this wrong, but how much do you know about carbs, carb linkage and choke operation? If you don't know anything about how the choke and the fast idle linkage works, you will need some help. If you do understand it, you should be able to fix the problem pretty easily. I can almost guarantee that it is one of the 4 things above.