What is the timing now? I would reinstall the dizzy aligned correctly with the valve cover off, re-time the motor with a light, and reset the idle speed. Install the "old" depression chamber for now. Let it idle and check vac advance with vac pump and light. Check centrif advance with vac removed/plugged with light and tach. specs:
Basic ignition timing at curb idle speed 7öBTDC (vac removed/plugged
Distributor
Federal
Governor (crank deg/rpm) Initial 0/1600\
Middle 9/2800 \
Final 19/6000 / centrif adv.
Vacuum (crank deg/mmHg) Initial 0/80
Middle 12/150
Final 23/280

Engine curb idle speed is 800 +- 100.

To reinstall the dizzy, remove the coil wire, then the cap and lay aside. Remove the valve cover. Rotate the engine until the timing notch on the crank pulley aligns with TDC on the scale on the timing cover. Check that both valves on #1 are closed. Rockers should be on cam low point and should be slight play in the rocker in the rocking direction. Dizzy rotor should point at about 10:15. Dowel pin hold in cam dizzy drive gear should be about 5 minutes short of 12:00 straight up. Remove the clamp bolt from dizzy stud and remove the dizzy. Align the drilled mark on the driven gear with the housing cast mark. Drilled mark is near roll pin hole. Holding shaft thus aligned, reinsert dizzy into the head, centering adjustment slot over clamp nut stud. Continue inserting held aligned until the dizzy gear begins to engage the cam gear and then continue inserting whith alignment hold released, letting dizzy shaft rotate as the engagement of the gears dictates. Dizzy rotor should point at the #1 plug cap electrode position, just about at the edge of the cap clamp. Reinstall dizzy and cap and coil wire, and set timing to 7*BTDC at engine idle of 800rpms. If necessary to set idle speed, make sure you adjust the right screw. The one further forward and angled a little flatter in the more inner position is the right one. Hard to see.

I'd make sure this stuff is right before I bought more than a new depression chamber. I've successfully repaired minor acutator diaphragm leaks with rubber cement thinned with alcohol, inserted thru a tube inserted thru the vac line nipple while holding diaphragm level. Let cure with diaphragm level at least overnight. Then repeat thru actuator end.

Over-advanced timing will kill top end and cause idle spitting thru the carb.