Did you put the aluminum ring into the wastegate flange where it mounts up? It should fit inside of the bottom flange.

If you did, have you tried some copper high heat gasket sealer?

I use the gaskets sold here: http://www.roadraceengineering.com/tial.htm (very bottom of the page)
The ones that came with my turbo kit burned up quickly and leaked like you said. These have not leaked yet and I also use the copper sealer.

hih