The brace for most of those Super Kodiaks is about 8". Anywhere from 7 3/4 to 8 1/2 will work, depending on the shooter.
I would be hesitant to use the newer strings, as you are dealing with one of the nicest Super K's ever made. It's a personal choice at this point since there isn't enough studies to prove yea or nay on new strings.
I'm using a 450+ on an old 1967 Shakespeare and it is working fine....over 1500 shots so far.
Get a new B-50....12 strand for that one. Order, or make it in 57", then twist it to brace. If you do that you will have a string that doesn't stretch as much as one that doesn't need the extra twists. At least that has been my experience.