The security of your steering system is so important that I recommend being very careful when making changes. I would rather use a replacement pitman arm that is one piece (but a little bit longer) than modify your existing arm. Is there any way one of your friends has a pitman arm of the size and shape you plan that you could borrow to check how it works. I also do not understand the geometry of the steering system enough to be sure if your planned change might have other (bad) effects besides the good effects you want you plan.