Spark Plug Change in 95 V8 4.6L bird

Hello recently I took a few of the old plugs out of my Thunderbird and they were horrible looking. It ran ok but backfired and was a little slow on acceleration but not bad. I went to the autozone and got some AutoLite AP764s which is the Plat version of the 764s that were in it. Now I got them to the right gap of .054 using a tool I got at the same time. I put them in the car and now it sounds like a fireworks show. I can't seem to stop it from backfiring. I took the plugs out made sure they were clean which they were I hadn't gotten anything on them. I checked the gap again and I think one was a little off I tried again and the same thing. The old Plugs have been in the car for over 5 years because it had sit for a long time. They should be good plugs and work even better than before. I don't know what to think. I have put the old ones back in it and they made it go back to way it was. Can my thunderbird not use the Plats or could it be something else. And I am sorry but check engine light didn't come on so I have no codes.

Thanks for your time and help.

You most likely have a intake or exhaust valve that is not seating completely which would explain the backfiring. There is nothing wrong with the choice of plug you decided on. It is backfiring more than before because of a hotter spark you are getting with the new plugs. whereas before it didn't have enough spark to fire the weaker compression. It would be a good idea to run a compression check on your engine. I'm hoping I'm wrong because this would be a major thing to fix. Maybe my logic is not up to snuff. Does anyone out there have another explanation for this back firing? If it is back firing wouldn't the cel show a misfiire?????

I would do a compression test if you have a bad valve the pressure will be low in that cylinder. Then pull the valve cover if its low and look for a cracked spring.
A gap in a plug wire will cause a backfire. This happened to my son and he dropped a sparkplug and cracked the porcelain. Use an ohm meter and it will pickup this type of problem. A visual inspection will pick up a cracked plug.