I am thinking of building a .375 H&H. Since the 7mm Mag has the same head size, can I build it from a used 7mm Mag? Will I have to modify the magazine?

What action are you planning on using? That may be a determining factor. But there is a very good chance that the action will work. But it will most likely require altering the rails and opening up the action. you will also need the proper magazine.

Am left handed so built one on a 7mmMag Rem 700 I had available. Didn't do anything but change the barrel.

SHortest answer, you want a 375 taylor, if you want the least mods...

send it to pacnor, wait 8 weeks, then can even match your barrel profile... or to les bauska, which will be a couple bucks cheaper

jeffe

I built my first .375 H&H on a Rem. 700 that was a 7mag. Just chambered the barrel, screwed it on and went shooting.
To this point it's never failed to feed or extract. It's very accurate and has taken quite a number of animals here at home. I've been very happy with it.
Pat