I just did this too. I bought a 4-1/2"? hole saw from home depot, and using a drill press, cut out the center of a stock cap, then i cut a 3"? hole out of the center of the cadillac cap to clear the dustcap and tack welded the stock dish to the back of the cadilac cap. Seems to have held up pretty good so far. Those hole saws were expensive. Let me know if you go this route you can just borrow it.
A buddy of mine did this for his 49 ford, I believe he attached them to a trim ring, so they will snap on to the wheel like a normal hubcap
A good trim ring can be tacked to the outside edge of cap,but remember the caps are heavy so make sure the tangs are really bent outward....