What is your total travel between centers?

Hence by kicking out your tail stock on the stock piece between centers with a dog leg driving the stock piece, this will allow for the cutter to run true on the  Z axis plain using the cross slide to the bed in a fixed depth (so you don't have to use the compound slide), but will produce a taper cut on the stock piece since it no longer parallel to the Z axis plain of the bed isntead.

Also, since you are going to be using a live center in the end of the bore, leave the barrel about 1" long, then after you taper profile cut, go back and face the muzzle to remove the live center peening to the rifling, thread and crown next.

Here you go if you need photo's,
https://rifleshooter.com/2015/01/how-to-taper-a-rifle-barrel/