the idle on my truck sits at around 1000 but when I put it into gear it drops to 400-500. I was wonderin if there was a way to get close to matching the two rpms

My truck is an 86 f250, 460, 1406 4 barrel Holley (brand new)

 

Couple of things come to mind. A stock 460 in good condition won't drop more than 100 rpm going from idle to drive. Usually engines with big cams (low manifold vacuum at idle) will drop more speed - but not 400 to 500 rpm. Something is effeminately wrong.

1) Timing. Do you have your vacuum advance connected to manifold vacuum? If so, possibly your vacuum is dropping enough when you put it in gear that your retarding the timing. You can try with vacuum advance disconnected (you would want to verify base timing is correctly set without vacuum connected)

2) Is engine running lean? Pull a spark plug as see if engine is lean ( is color white) also an excessively wide plus gaps or worn out spark plugs/wires can cause idle speed problems.

3) torque converter problem? You can do a "stall test" and see if there are any signs of converter failing.

4) Does your truck have an EGR valve? If so, maybe it's leaking some.

5) Did this problem just start after you replaced the carb? Need more info on this.

6) Did you remove (leave off) the idle speed solenoid or dashpot when you replaced carb?

 

Is the carb maybe stuck on a fast idle cam?

 

I bet this is the problem. If it works, find a port that does not have vacuum on your carb unless you rev the engine. It's usually the one on the front pass side on a holley(up high sticking out toward the pass side fender). If you can't find one without vacuum, you throttle is open too far, you will need to slow the idle down till it quits having vacuum on it.