Almost all 300 Win Mags and 7mm Rem Mags are going to be on the small/standard shank. If the donor gun has a sporter barrel it is almost certain it is a small/shank. Again there is always the exception to the rule, so to be certain look for a noticable step/shoulder on the outside of the barrel just forward of the nut. If there is a step its a Large/Target Shank. You can also compare the nuts themselves. At the gun shop, grab a 243, 223, 308, or 22-250 with a factory sporter barrel. Those I do believe are always small shank. Look at the length of the nut on that one and compare it to the gun in question. A Large shank nut is shorter.
Hope some of that helps.
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