most short mags (wsm) will work you may have to work on the feed lips also
a good way to check is to measure your mag well and the overall cartrige length you would like. 7mm wsm
,300wsm,325wsm all come to mind as builds i have seen.
rebarreling is easy even though you may want to take it to a gun smith to get the barrel off with to start. they torque it down to 100# at the factory. when you install a new one all you need is 50 to 60 # (helps if you want it a switch barrel) tools can be found at brownells or savage shooter supply. there are two types one for smooth nut one for for large/small shank. mine cost 40$ at sss.
when you reinstall your barrel you'll need head guages one in "go" and one in "no go" or the laymans way is a regular shell for a "go" add 3 pieces of clear scotch tape to primer end of shell for "no go" .
screw the barrel in on the "go" untill it stops agaist the bolt head, hand tighten the barrel nut, check with the "no go" if the action will close tightn the barrel alittle more against the bolt until it wont close then check again with the "go" to see if it will still close. next with the "go" guage in place in the bore tighen down the barrel nut. it helps to hold the barrel with something so as not to tighten the unit as a whole and have to start over (also were steel guages come into play instead of brass)
if you plan on taking your barrel off yourself make sure not to twist the action. i made a simple tool out of the original stock (has custom now) by cutting off the forend at the shank so i could fit the tool around the barrel nut. i then locked the stock into my bench vise and used a plastic dead blow to loosen the factory nut. some people use two blocks of wood and a vise for a BARREL VISE (do not do this on the action you may twist it) to do the samethig however i found this alot harder than the stock trick.
its alot of fun and satisfying when complete. good luck
any questions along the way the good people at savage shooters forum can really help. some there have 300 ultras on short actions but they are single shots and require removal of bolt to load/unload