86 beauville van

I bought an '86 Beauville van which runs very rich and stumbles at takeoff. I replaced plugs, wires, cap and rotor. I also searched for, found and repaired all vaccum leaks. When I pull off ignition wires to identify a problem cylinder, all cause a decrease in RPM except #8 and #5. In fact, with these two wires unplugged at the same time there is no difference in engine idle ( plugged or unplugged ). It's a 350 (5.7) with a rochester Qjet and about 200,000 miles on it.
Any ideas or suggestions?