before I explain the problem let me detail you on my engine so to better grasp what I may need.
350 block, bore 30 over, 400 crank, HEI ignition with DUI cap, rotor and coil, taylor wires with ngk plugs gapped at 50, non ported iron vortec heads with 520 lift springs, edelbrock airgap intake, holley street avenger 770 cfm carb vacuum secondary electric choke, headman headers 1/58" 3"collectors to 2.5" true duals dumped on magnaflows, 224/230 extreme energy comp cam hydraulic flat tappet.

now on to my problem: I recently replaced the headers and old Y-pipe single exhaust over to a larger diameter headman shorties and true duals, after that the car runs like if running on 7 cylinders real crappy. I checked compression on each cylinder which was between 195 and 200 psi, I then tried tunning the carb idle mixture screw to no avail. I am thinking maybe its ignition. should I run a mallory or Msd box? and which one to make a better spark which to date the engine is always too rich no matter what I do to the carb. keep in mind I am not running any sofistacated EFI or computer or nitrous so I may need just something to create combustion with more efficiency.
your advise is greatly appreciated.

 

If it were me, and it were my car, this is what I would do. First off, check the plug wires and make sure they are in the right order. If they are start the car, and with a pair of insulated pliers, take the plug wires off and on 1 at a time, and see if one of them DOESNT make a difference. If you didnt change anything else but the headers in the engine compartment, I am guessing that the plug wires were removed so you could install the headers. Odds are you have the firing order wrong, or in the R&R process one of the plugwires was damaged, this happens ALOT. Give it a shot and let us know whats up.