Exhaust leak

Installed DT Long Tubes with motor transplant. Everything went fine as far as the header install goes. BUT. My truck sounds like its 20 year old beater v6 now. It sounds OK for the most part but I can definetly hear some mad exhaust leaks.

First off. When installing the headers I did re-use the manifold gasket as per instructions. It's a 2 layer metal gasket which is supposed to be ok to re-use, so I was told at time of purchase. Just kinda seperated the layers a bit and cleaned between them as best I could. I think my leak may be coming from here, or at least one leak. But how do I find out for sure??? Just feel for hot air puffing out around header flange? Or is there a more accurate way?

Second. when I started the truck today and got under it, even just a idle I could feel a leak around the drivers side header/Hi-flow cat flange connection. When I installed the headers there was only a tiny 1/16 inch thick, round copper ring type gasket that I had to re-use between the hi-flow cats and the new headers. So today I un-bolted the headers at the hi-flow cats and used some High temp exhaust silicone. The Hi-Flow cats have what seems like extra pipe that sticks out past the flange face, I would guess to go into the end of the header and make a better seal, But wonder if that isn't leaving potential for the leaking? After I silicone it, think it may have helped a little, but still hear the leak. And after the amount of silicone I used, im pretty certain its a bad header flange leak. So now if Im gona unbolt the headers to install new gaskets, is there a better type of gasket to purchase?

And also to any others with headers, Do you think it would be beneficial to use the extra stud/bolt holes that are on the motor, to help secure and seal the headers better?

Any Advice???
For a truck that has like 19k miles on the motor, brand new Long Tube headers, brand new hi-flow cats, new custom Y pipe and muffler, and Uprev, it sound like a piece of junk that has a broken exhaust rather than custom. :censor: