Need tips on radius cutting rear wheel wells

I have a 72 dart custom and I want to put on a my bigger set of slot mags from my old dart sport i have for it, but they are too big to fit in the rear. Narrowing the rear end and getting tubs are out of the question for me, I want to give a shot at radius cutting. My main problem is I don't have a plasma cutter, I only have a saws-all and a grinder. Its my first time trying anything like this(pretty new to the car world still) so I was wondering if anybody had some advice for a newbie like myself trying this. I am set on going for it, I just would like some tips on drawing a perfect radius and cutting it out without it looking like a total hack job. Thanks in advance, you guys are always awesome on here!