Re: holley 750 too much for 383?
The running on is not a sign of running rich. It can actually be a sign of running to lean. Probably caused by your reduced jetting. This can cause hot spots in your chambers that act like glow plugs (hot carbon), this keeps the car running. The idle smell has nothing to do with your jets. They are two different animals. The first thing I would suggest is putting the jets back to factory. After that pull the carb off and turn it over, on the primary side there are two slots in the base plate (one on each side), you need to adjust your throttle blades/butterflies so that that slot looks like a perfect square (looking at the carb from the bottom). This is your transfer slot, if you open the throttle blades too far (trying to increase your idle) you uncover too much of this slot and it starts allowing fuel from your main circuit in at idle, this gives you that gassy smell at idle. If you can't get the idle where you want it, you will need to open up the rear throttle blades a little to gain the rest, you can't just crank open the primary side (common mistake). Once you set the throttle blades then adjust the air mixture screws for either highest vacuum or highest idle (usually the same), then you can adjust your idle scre down from where it was set before (from the bottom), you do not want to have to increase the idle there, if you have to then try doing it with the secondary side.
I am sure pictures would help, I don't have any, I think there may be a few floating around here if you do a few searches on tuning a Holley or something like that.
If you still have a problem with run on after that, you may want to re-check your timing and adjust as needed, I doubt you will have a problem though. I am surprised your car isn't lean surging when cruising, you took the jet size down quite a bit.
Hopefully I explained this well enough to follow. If not just ask questions. Someone else might be able to draw a picture with words better than I have.