Re: 581 Barrel removal?
Can't go wrong with GM rimfire barrels in my experience.
As to which barrel blank, you might give them a call and see if they have any barrel blanks that have been center turned and cut for their 10/22 bull barrels (but not chambered or had the stub/shank cut). Even with the broached notch present, the barrels can be easily fit to the 581 so that it is hidden by the stock. I've done this on two 581's. Both came out nice and shot well.
That 25" blank would work (I have used them as well on other projects), but if they have something already turned and cut to 20", why waste the machine time?
JMTCW...