One sugestion, is to get a little vise, clamp that on the drill press. Maybe take a piece of frame rail, either a small scrap, or the end of one, and test a couple holes, to get it right where you want it to be, up and down the rail, Then make sure you don't move the vise while your drilling, this will make sure every hole is in the correct position. What I did, I knew how long I was gonna make mine, then added a couple inches. Drilled a hole, and put a short 1/4 bolt in it, with a nut on the back, so the rails would stay nice and tight.. This will make sure they stay inline while you are drilling the holes... Then once you have all holes, and ready to assemble, you then cut off the 1/4 hole piece, and whalla...
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Ignore the periods, the spaces go away, the flat bars, are kinda like how you need to put the frame back to back, so they are aligned...
One way to maybe help see how long you want them, is to take a flat bar, and drill out some of the holes, to make one side of the frame, and attach the leaf springs to that, and see your different wheelbases, or if you know you'll never use your other frame, you can put it back together, and cut it, then move it around till you like the length. But then your ruining any chance of redoing it, Or maybe just add a piece to the center, and you woun't have to drill many new holes..
Just take your time and be precise..
Thanks
Cody