OK, here is my take on this modification. Don't drill the ram, but mill a Vee in the front instead. One mill cut straight across ,and perpendicular to the Vee, and where the tool will push against it. By using a Vee, and a clamp it will accomodate tools of various diameters, eliminate bushings for small ones in big holes, and in general makes like easier. Go to MSC's website, and look at Vee blocks to get an idea on what I am talking about. Of course the stamps won't all be centered depending on the diameter of the stamp handle, IE; smaller diameter will sit farther back than larger ones. but that is no problem in my eyes.
My background is that my Dad was a machinist all his life, Mom milled out radial aircraft engine master rods during WWII, and I have been machining since 1985. I am currently the tool maker for a CNC screw machine shop where I make, and modify tooling for the production floor. This is not a hard job for even a small basic job shop to do. Replacement Vee block clamps are available, we get replacements for ones that get broken on the floor at minimal cost. If you can't find a local shop that is willing to do it, I am just a USPS flat rate box away =) I like projects like this, and I even barter.