305E Backfire out Carb and Exhaust

[boycec1]

My son and I recently bought a 1965 GMC C20 with a 305E. In an effort to troubleshoot a low compression issue on cylinder no. 1, I ended up adjusting the valve lash on no.1 and the rest of the cylinders. Turns out this didn't help no. 1 and it made things worse.

After the lash adjustment we now have a nasty backfire out of the right side exhaust and the carb. It idles fine but when I rev. it our try and drive the backfires begin.

I have double checked the lash many times and i confirmed that I have .012 on the intake and .018 on the exhaust. I checked this by manually turning the motor and waiting for the valve to fully close and the other one to start to open before I checked the lash.

If I pull the no. 6 plug wire the backfires go away so I am pretty sure the issue is with no. 6.

Do any of you guys have a suggestion on what I try next? The really frustrating thing is that we drove it around the block before we started down the valve adjustment road and it ran fine. I have been kicking myself all day as I am pretty sure I screwed something up but I can't figure out what.