I found this info on one of the Gun Auction Sites. It had all of the specs for the Guide Gun listed in the ad for the auction of 673's.
Caliber Barrel Length Twist
6.5mm Rem Mag 22" 1 : 9
308 Win 22" 1 : 10
300 Rem SAUM 22" 1 : 10
350 Rem Mag 22" 1 : 16
The Guide Gun is now out of production, and will likely never return in some calibers, if at all. You can still find them at reasonable prices if you want one. I have one in 6.5 Mag and I am getting a .308. I plan on shooting and hunting with both of them.