I do not think the long actions have changed. Get on Boyd's site and they tell you to measure the distance between the action screws on a short action. The newer feed systems have a bit longer distance between the action screws on the short action. Below is taken right off of Boyd's site. This should explain the differences in the short actions.
"OUR STOCKS ARE BUILT FOR THE LONG ACTIONS (ALL) = 5.062"
AND THE 2ND GENERATION SHORT ACTIONS (1998-2006) = 4.275" STAGGERED FEED MAGAZINE
WE DO NOT AT THIS TIME MAKE THE STOCK FOR THE 1ST GENERATION SHORT ACTION (1958-1987) 4.522" OR THE 3RD GENERATION SHORT ACTION (2005 AND UP) = 4.40" CENTER FEED MAGAZINE"
The distance between action screws for the stagger feed is 4.275" and the newest straight feed is 4.40". All you have to do is measure the distance between the action screws to find out which stock you need. As the newest actions become more popular, Boyd's will make stocks for them. They make very good laminated stocks BTW I suspect they under contract to make the newer stocks for Savage exclusively right now.
Good Luck and Good Shooting