I think I've got them seated all the way down w/o crud in there. Some of the headbolts came out clean, but some were nasty, so I knew I'd have to do some work on them. Brake clean, compressed air, grooved headbolt, tap were all used. They all seated to the same height, finger tight per instructions.

.009" was removed from the cylinder head surface, and any amount removed from the head could decrease the clearance to that ridge on the inside of the valve cover. So maybe that's part of it.

The Celica GTS thread is here, maybe it's just a valve cover design difference. The thread indicates an issue w/ the later valve covers, but I don't know what that date range would be. I'd say 85-95, but since you/4Crawler didn't encounter this, I'm not sure what to say...