You need to see what is keeping the throttle open or causing the high idle.

Check the throttle cable adjustment/transmission kickdown linkage. Is the cable adjusted long enough/linkage not binding and allowing the throttle to close fully? Disconnect them and see if the idle drops. Since you had the carb(s) off, something may not have gone back together properly.

Check to see if the fast idle cam/lever are touching at idle. They should not be touching when engine is warm and choke is fully open (choke plate should be vertical). Next would be the idle solenoid. Disconnect it and see if the idle drops. If the RPM drops, adjust the solenoid. If not, check the base idle screw and see if you can lower the RPM. Check for vacuum leaks.

If none of these things work, check to see if you have manifold vacuum to the vacuum advance at idle (disconnect the hose). If the RPM drops, check the hose connections. You should not have manifold vacuum to the advance unit at idle. If the hoses are connected properly and you still have vacuum to the advance at idle, the throttles are opened too far.

Did you just remove the center carb or all of them?

Take the carb(s) back off and look for anything hanging up the throttle plates (i.e.; throttle plates rubbing on the throttle plate bores in the base, base gaskets hanging into the bores, bent/binding linkage, etc.). If the throttles are open too far, you'll be into the transfer ports in the carbs, not just the idle circuits.

If it idled well before, go back over each thing you touched one at a time...and look carefully for any problems.