Re: Metal mouthpiece for a vintage Conn?
I recommend large chambered mouthpieces on the old Conns. Some of the modern small chambered pieces (like Selmer Paris metals) tend to run sharp - though the Selmer USA metals might work ok.
If you really want to try metal, I recommend Otto Link SuperTone Master or Guy Hawkins - like a 6 facing on either. Vintage large chambered mouthpieces like Dukoff Hollywoods could work great, too, but they sell for a premium.
As you are used to hard rubber pieces, you might want to stick with hard rubber, though. Vandoren Jumbo Java mouthpieces give a bold, edgy, sound but they would be far more familiar to your embouchre than going with a metal mpc. A 55 facing on one of those would be a good size to try. I've used the Vandoren Jumbo Java on my old Conns to good avail. I've got a 55 and will soon have a 75 - not sure which one I'll want as my keeper.
Those are great pieces you've got to begin with!