Are you running the right plugs and gap for the MSD6a? I was having somewhat of the same problem in my '74 Nova. I replaced the plugs and set the gap to .035 timing was right but it didn't run right. Then I noticed I had a HEI distributor, which didn't come on the '74's, so I changed plugs and set gap at .065 now it runs great. If your distributor isn't OME run plugs and gap meant for it not the truck.