I was going to post a mandrel is useful so long as you can ram it in the press. Just taper the top of it and use plenty of grease. I used a mandrel to form my .45 WinMag bodies from 1x2" tubing. Unfortunately my magwell requires some tweaking to get a better mag fit but the mag itself works successfully after mandrel-izing it
Question - I've successfully used my 9mm Uzi mags with my .40 cal Uzi barrel conversion. Capacity is the only thing affected. That being said, if the magwell on your MP5 will hold 9mm magazines, you should be able to recruit those into service. I haven't tried stuffing .40 cases into the MP5 9mm mags... does that work?