Sounds like she may be running lean and the added load when in gear is bringing the idle down. Not for nothing but I recently bought an Edelbrock carb for my stang. Right out of the box the float was sticking, she stumbled on hard lefts and had a dead spot off the line. I took it off, put it back in the box, and made the store take it back. I opted to pay the extra and got a Holley Street Avenger instead. I've used nothing but Holleys for twenty years and I guess I'll be using them for the future as well. I'm going to try and post a shot of the new carb in my photo album, I like the features of the Avenger carb. Tunable secondaries and no more power valve blow-outs. Good luck with yours, I would start with richenning her up a bit and see what she does, recheck timing, and make sure there's no vacuum leaks. See if you can improve it by adjusting the air/fuel screws on the front of the carb before you go rejetting.
BUILT FORD TOUGH