You are going to Sub-Arc inside a 6" pipe?
We do a heck of a lot of SAW in our shop but I got to admit I never would have concidered SAW for inside a 6" pipe.
What Make and model machine are you using? We use mostly Lincoln and ESAB. As far as a 45 degree head goes I dont recall ever seeing one. For out of the "Norm" stuff we have to get pretty creative at times.
You can e-mail me if you are interested in hearing more.