I had my flange cut from mild steel at a local steel shop. (actually lockjaw did ) He traced the gasket and the shop cut it out. It's a better way to go since Steel is stronger than cast flange like the stock one and it's easier to weld steel to steel than cast iron to steel
Have you had a 18 wheeler drive by you under boost when you have your windows down? Well, that's what mine sounds like I'm running thinnest gauge steel pipes (16 or 18 ga I think) on DP and the charge pipe. Plus 12 inch long K&N filter.
I like my setup with HKS 2mm and head studs. Price isn't much different either way you go. I guess the benefit for O-ring is that you can replace just rings if you broke anything.
It's prolly the hose or the clamp going bad. Mine was leaking like you discribed but after replacing the hose and tighten down the clamp, it's fixed. I wouldn't remove it if you don't have to