As has been suggested, I don't think you will have any trouble unless you place the lock high. I would rather chisel a slight groove in the barrel than cut down the mainspring. It's a simple matter with a radiused die sinkers chisel. I'm not sure about forged springs being thinner than typical cast springs today. In my experience I haven't drawn this conclusion. A final note about the finishing of springs... Like most others who go to the trouble to make a spring, I finish them pretty well. In looking at original springs, this is typically not the case. Our concern about file marks isn't shown on the originals I've examined. I understand the rational behind longitudinal small tool marks, but perhaps it isn't as critical as we might think. Even springs by one of the best 17th century Paris makers I've examined show visible file marks at an angle to the length of the spring. This is on a top quality lock. Just some things to consider.