I presume you are running a carburettor and offer the following;
you may have a small vacuum leak or carb problems but have a read:
its not uncommon to have a rough idle when the engine is engaged in a gear(thus it has a load on it); pending your camshaft profile/engine vacuum available versus brake booster requirements. Often if the engine vacuum drops too low and the carburettor is on the rich side the engine will slowly start to bog down but having the trans in Park or Neutral is basically the same because in both cases the fluid is just pumping through the bypass ports so the idle should really be the same whether in Park or Neutral and the problem should only arise when you are idling with a gear engaged. The only way to check if this is vacuum related is to disconnect and plug the vacuum ports at the manifold(& bottom of carb) and see what happens. If the diaphram in your vacuum canister for the trans is faulty you should get a thin whisper of white smoke out of the exhaust if you hold the revs at around 4500rpm for a minute or so and give it a rev. The common cause of a rough idle with the trans in gear is low engine vacuum due to camshaft/brake booster demand versus the stall speed of your torque converter(the engine rev at which the converter starts to lock up and drive the wheels). If you are running a carb with secondaries; you can open the secondaries up a touch to let more air into the carb and then drop the idle adjustment to bring the engine revs back down. let me know.