HS Header, O2 Sensor

Does anyone know if the O2 sensor adaptor (that is supposed to be welded on after the HS header is installed to keep the check engine light from coming on) is a must right after the header is installed?

I'm doing the header install myself but I need a shop to weld that piece on for me. I need to know about how much time I have to get it to a shop!

I don't want a loud fat fart can for muffler. So I'm plan on buying a 5Zigen fireball muffler then have a shop do a cat-back with that.
Please give some inputs on that. Thanx.

its really not that important, its just annoying to have that light shining in your face. So you should get it done as soon as you can (just in case something really does go wrong), but it won't hurt to go for like a month with it like that. If it were me I would try to get it done within a week or 2