The powder charge of a 375H&H is usually in the 67-75 grain range. The bore capacity however is far in excess of a 30 cal magnums, which burn about the same charge of powder.
The 30 cal magnums really need 24-26 inches of barrel length but the 375 does just fine with even 20".

Don't worry about the shorter barrel. The longer 24-25 inch tubes look nice and give a longer sight radius if you shoot the rifle with irons instead of a scope, but for use with a scope you'll be fine with a 21" barrel.