My son and I made a project of installing a set of Mac headers on his 95 Z28 this weekend ... He is home for Spring Vacation from college and this was an opportunity to spend some quality time ... We got them installed ( some 20 hrs later ) including new plugs and wires ... The problem we encountered is that while the original exhaust manifolds mount points had an inner lip that the airtube compressed against, the receptacles on the headers are totally hollow so that the air tubes never really get tight and they leak. I am thinking that maybe I could screw some kind of hollow plug into the tubes to create a compression point so that the airtubes will seal. However, if someone has a better idea, or, know of something thah already exist explicity to address this problem, please let me know. Hey, it was fun weekend, but, I am am glad that I have only 1 son ... My hands do not want to do this again any time soon ... Thanks in advance .... Steven Sr.

Your on the right track with 'maybe I could screw some kind of hollow plug into the tubes to create a compression point so that the airtubes will seal.'
What you need are the "tubes" themselves. They can usually be pressed out (or helped out with light hammer blows*) of the original manifolds and inserted in the holes in the headers. You insert these tubes into the header holes, then reinstall the airtubes and they will seal.
The inner air injector tubes are also available from the dealer if you can not get your old ones out.

* note: Be very carefull removing the old ones. If you hammer on them it may collapse the ends or bend the tubes. If they do not come out with light blows, go to the dealer or a good local auto parts house and get new ones.
I have seem these at several of the local parts houses like NAPA.

Sounds like you need an adapter to go from 1/4" pipe thread to 5/16 flare. I'm guessing the sizes but that is probably close to what you need. There are two types of flares. One is 45* known as SAE and the other is 37* known as JIC. Figure out which you have and see about getting an adapter. I would recommend steel for this application. Most autoparts stores will have adaptors but many will be brass. I don't know how well Brass would work.

I usually reuse the original little tube extensions. If you soak them down with WD40 or something first, and then sand the carbon off with scotchbrite they will usually come out. They are still availiable from GM, but they are expensive about $9 each ( canadian ).
So the headers do not have the little lip the extensions sit against?, and then the smog tubes are installed on top of them?
I also usually cut them down a tad to say about .500 long, it seems to help the backfiring situation on older A.I.R setups.