Kickdown setting for FMX

if the early 4300 makes things happen you could 'recurve' the distributor to operate on manifold vac the same way the dizzy that came paired with that carb did. 

setting that up will require an adjustable vac pod on the distributor to limit the vac input to about 20* total at idle including the initial timing setting IIRC

OK i found the post i like to forward

The vacuum advance unit is adjustable. You need to limit the total advance under vacuum. You adjust the unit by inserting an allen key (I do not recall what size, try several until one fits) into the front hose nipple. Turn it one way or the other (been a while, obviously) until total additional vacuum advance is about 10 degrees. Your engine will like this.
In my daily butt hauler I run manifold vacuum advance. The car is a 1973 Mercury Montego four door. It's a pretty big car, about 4200 lbs. The engine is a 351 Cleveland 2V, it has a PAW/SSI 11339 (194/204 @.05, .458/484), a Performer intake, a 600 Holley and duals. I use a Pertronix Igniter in a Ford dizz with a Ford Duraspark coil. The Holley has a light sec spring, so it cuts in quickly.
I run 12 degrees initial advance, use the stock curve and use manifold vacuum for the vacuum advance unit which is adjusted to provide only 10 degrees additional advance. The result is that at idle, the engine runs smoother with the ac on (York comp that does not cycle, unless you count when I have to turn it off when the core freezes. I could store meat in that car). The vacuum provides an increase in efficiency at cruise (18 MPG with 2.75s). Being that it is manifold vacuum, the amount of additional advance is reduced when I push into the throttle. Acceleration is easy and effortless, especially in the 2-5K range where Clevelands run great. The greatest advantage of this set-up is the reduction of advance under acceleration. The US 2V Cleveland heads have an open chamber. That chamber coupled with the dished pistons of 1973 create a great deal of surface area. Detonation is a real problem here, but I am able to run the car on 87 octane with no problems, even on hot SoCal days with the AC on. When I was trying ported vacuum, I could get it to run well but detonation on acceleration was a problem. As it is, the car runs very well