Make sure you use good gaskets for the uppipe. The crosspipe/manifold to uppipe joint is an area that is prone to leaks with a solid pipe. I used copper gaskets in my latest project, but you don't have to go that far. I would recommend some of that copper gasket spray though, it helps with heat transfer. I have about half a can leftover if you want to use it, and I did all gaskets from the heads back to the DP.

BTW, if you haven't replaced your downpipe, I would recommend doing it at the same time. You have to take it off anyways to get to the uppipe bolts. Blah blah blah, yadda yadda yadda. You've probably heard all that before though

Make sure you retorque the bolts too after a few days of heat cycling, then again after a few weeks. I've had them fall out. Seriously. I lost the bottom 3 nuts off the downpipe studs.