I had a problem after I first installed the sidepipes with the slip joints leaking. CA Cobra told me to cut the weld joints on the inlet side of the sidepipes for a better fit when I first installed them. They went on too easy the first time. I used Exhaust System Sealer (ceramic putty or mud), available at any Auto Parts store. All the sealer blew out when I first started her up. I did get a little backfire here and there, but not much. I was getting soot marks on the pipes and the air cleaner was getting black.
I removed the pipes and used a tailpipe expander and expanded the header pipes for a tighter fit to the sidepipes. I used an exhaust coupler the same size as the sidepipe inlet to use as a guide for sizing the header pipes. That way I didn't have to remove the headers or keep raising and lowering the sidepipes for fitment, Too heavy! Don't make them too tight! Just enough to slide inside the guide freely.
For torque, I used a 1/2" breaker bar and the correct size socket for the expander. It does take a bit of time to work each pipe in small increments (depth) until you get the desired size. Don't try to muscle the expander with one try, it will break the expander or you slip off and damage your car and yourself.
Again, I used the same sealer. I put plenty inside all pipes and had to use a rubber mallet to get the pipes all the way on. I also put some sealer on the outside of the joints. I used a turkey injector with the needle end cut off flat to inject the sealer as much as I could, then used a popsicle stick to push it in around the joints where the needle couldn't reach. I don't have any more leaks and it does sound different with no backfires!
The sealer doesn't cure all the way thru until you run it and heat it up. I found that out a week later! It blows out any excess moist sealant.
Some of the above suggestions were given to me by Frank (Backdraft), Armond (CA Cobra) and the rest I figured out myself.
Hope this helps.