mine had the y pipe removed and was duals, but I had manifolds in their place and when I removed them for shorties they bolted right up, no leaks at all, no lower gasket and IMO the ONLY way to go with them.
I used the hardware that came with them, and only had bolts back loose twice, once was a head side bolt, no issues just noticed it was loose. The other time was after I had the pipes off and put em back on, started driving after a day or so I lost 2 nuts on the collector of the PS pipe, maybe I just didn't tighten them enough.
Only thing I had to do myself was make a spacer to fit my ps or alt. bracket up front, I just used an old socket that was cracked. They come with them but none was the right thickness or hole sze for what I needed.