Exhaust Ahead Gaskets

Car: 73 Duster 340 stroked to 416 with TTI headers.
Today I installed new copper header gaskets replacing 2 year old aluminum gaskets that were leaking. When I finished I started the car and the exhaust leaks were worse that with the gaskets I replaced. I also used Stage 8 Locking Fasteners which I have heard nothing but good comments about them.

I'm looking for advice. What brand and type header gasket material does the best job?
Any advice, tips ya'll can give me would be greatly appreciated.

Thanks, Doug