I have made many a pattern like Kevin's example. It works well. I few years back I bought a drill press with a round table that turns. Since then when I need to drill a bolt circle I chuck up a scribe in the drill and mark the center of my circle. I then swing the drill table to the side half the dia of the circle and lock the table down. I just lower the chuck down until the scribe hits the plate metal and spin the table. This give me a perfect bolt circle. If you use a thick plate, you can drill and tap the holes for your pump mount. This way you don't need to weld studs to the plate. The scribe provides a location for your center punch and you can remove the scribe from the chuck and install your drill bit and have your bolt holes align perfectly. I tend to get a little off with my drilling using the cardboard templet. At any rate, mark you engine mount and your pump mount before unclamping the metal from your drill table and the pump and engine should line up without any shimming or guess work and no welding that can warp your mount.