GTO spark plugs

[68conv.driver]

I have a problem. I know that my 68 GTO is supposed to use AC Delco #7 plugs, but they don't fit. I recently rebuilt the motor (dropped valve) and found that I've been running the wrong plugs. I tried to put in the correct ones, but the outerbody of the plug won't fit into the head. It will go in partially, but the threads won't go all the way in. The heads have a 6X on the center exhaust port and a J245 near the back. The block number is 9790347. The car hasn't ever ran up to my expectations, but I'm not really sure what it should do. The plugs I had in it that fit were AC Delco R44T. The outer body of them is much narrower and fit into the head. I had a NAPA guy convince me they would work. Thanks for any info.

[MrPbody]

Your 6X heads are '76-'79 400 or 350 (Pontiac) heads. We use AC R44TSX. Order them for a '76 TransAm with 400 engine. That will do it. The '68 heads used the larger plugs with washers. The later are "tapered seat" (TS) plugs.

Jim

[MrPbody]

Your 6X heads are '76-'79 400 or 350 (Pontiac) heads. We use AC R44TSX. Order them for a '76 TransAm with 400 engine. That will do it. The '68 heads used the larger plugs with washers. The later are "tapered seat" (TS) plugs.

Jim

[68conv.driver]

Thanks. I had put in R44T plugs before. Does the "T" mean tapered too? Because they don't look tapered and the heads do. I saw also that some guys are using RT43TS, RT45TS, RT45TSX, and autolite 666. Is one better than the others. I was wrong about the block #. It's 9790071. I guess that's out of a 69. The web told me it means a 69, 400, 330HP, Auto, 4BBL, FB. What does the FB mean?
Thanks