It's a good idea to get it done all at once, both the threading and cutting.
I think the 26" is a bit too long myself, so a 24" or even 22" length would get good in my opinion. They only thing you will loose is a bit of velocity, but probably not all that much. You may see an increase in accuracy, since by cutting the barrel, you are in a sense making it stiffer.
I was doing some reading on the subject awhile back. There's some good info on this page - scroll most of the way down:
http://www.chuckhawks.com/rifle_barrel.htm
There is probably a mathematical formula that could be used, but I don't think it would be a simple one!
I think the 26" is a bit too long myself, so a 24" or even 22" length would get good in my opinion. They only thing you will loose is a bit of velocity, but probably not all that much. You may see an increase in accuracy, since by cutting the barrel, you are in a sense making it stiffer.
I was doing some reading on the subject awhile back. There's some good info on this page - scroll most of the way down:
http://www.chuckhawks.com/rifle_barrel.htm
There is probably a mathematical formula that could be used, but I don't think it would be a simple one!