Chevrolet: 350(5) chevy, missing bad., angle valve job, loose plug

Question
The whole dist (cap too) is new from Jegs-42,000 volts)  Block was decked, heads have been milled .20. I am just beffled as to why when you pull the #8 plug wire, it gets better. A friend said that most likely #5 or #7 have a problem ie. the nut loosened on the rocker and it's trying to shoot gas over to #8, flooding it out and causing the backfire. I currently have the valve covers off, and am wondering where to go from here. ps it has a 3 angle valve job.k5k22
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Followup To

Question -
Hi,
We have a nice 81 El Camino and have no luck with 2 and now the 3rd engine is not sounding too great. Just had a .30 over engine put in to replace the 350 that overheated and broke some ringe scoring the wall bad. When we got the car back it had a miss but the person who did the work showed that 2 outlets on the manifold were cooler than the others (with a hand held laser devise) he said the plugs were bad. We bought plugs to replace them today, and they sold us the wrong size. But, I checked each plug, by removing the wire to see if it made it worse or not and only one worsened the performance, I cleaned that plug and she ran ok. We went to town and washed it. It then started caughing and backfireing etc. I had a loose plug wire, it didn't even help plugging it in. However, when I pulled the plug wire closest to the passanger it ran better. The wires are correct... Please help. lovejesus@centurytel.net

Answer -
Tim;
 Hi,  Did you have the heads done also , if not I wonder if the valves are ok,and did you gap the plugs when you put them in.  if the wires  are old it can cause  a miss . or  the dist cap, , timeing, etc  check the wires on the cap  also , it should be   1-8-4-3-6-5-7-2 .and check the inside of the cap,rotor also . see if that helps.
Roger get back 2 me ok

Answer
Tim ,
  when it is dark out  with the hood up look if you can see any arch from the wires, and with the valve cover off do  the springs all look ok on that side. and the push rods.also did you re check to see if any wires are crossed. it could be a lot of things the pain is doing all of the checking for it.did you try to replace that 1 wire ? did you look for a vacume leak on that side from the intake ?