This is my very first time on any forum or blog, please have patience.  The following problem took place on a 1967 Plymouth Sport Fury, 383 with a four barrel AFB.  While on a route 66 trip recently, just outside of Chicago, we lost cylinders number 6 & 7.  We first tired eliminting the electrical problems by replacing the distributer cap, rotor, and both plugs and wires.  NO DIFFERENCE.  We insured the timing was correct.  In doing that task we discovered the engine ran better when we left the vaccuum advance hose off and plugged.  Although the engine ran smoother, it was still very noticable we were only on 6.  We took off the upper carb cover plate to insure no debris had made it past the filters.  We drove it back to Fort Worth still running on 6.  Since our arrival back home I've removed the valve covers to determine valve action, they appear to be working properly.  I did a compression test, lowest 118, highest 140.  According to the book that's very close to the limits.  However, the 118 reading is on the number 6.  I've checked for vaccuum leaks as well as I know how (carb gasket, intake ports, and power brake).  I need some help, please.  

If everything is mechanically OK.  Valves opening the right amount, timing, etc.  The only thing can be either no gas or no or WEAK spark.  Have you tried a couple of new spark plug wires? Have you connected the spark plug wire on cylinders 6 & 7 to a couple sparkplugs that are known to be good and turned the motor over to see if you have spark? You should see a strong blue/white arc.  You may have a weak coil but I don't think so. If you have spark, then it has to be fuel.  You may need to remove the valve cover and try and measure how much a good cylinder (rocker arm) is opening the valve (should be about .4 - .5") vs the bad cylinders. Since you stated it ran better without the vacuum chamber connected, I suspect a distributor problem, points, condensor, etc.  

Thank you for your comments.  I have changed both plugs and wires for 6 & 7.  No improvement.  However I did not do the outside of the cylinder trick with the plugs, that's a shot I'll take soon.  Also I did not actually check the throw or travel of the rocker arms.  Not a bad idea, I'll also give that a shot.  I've checked the points and condenser, and changed the coil.  No change.  I'll try the rocker arm measurement and plug trick.  Thanks again.  

I live in texas too lol, My 400 was doing the same exact thing and i trieed everything to fix it except look in the distributor.... The points were so far off that I am suprised it even ran, after I adjusted them to .016 it ran pretty good. just trying to help