How do you convert your 390 classic car engine that uses regular gas over so it can take unleaded?

The problem with no-lead is higher cylinder head temperatures. Such high temps are known to warp the thin valve stems of lead burning motors. The fix is to remove the cyl. heads and have them rebuilt using the thicker stemed sodium filled valves. This requires drilling out and redoing the valve guides and purchasing all new valves,seals and a VG gasket set.

As such means a valve job any way, you might get away with not doing anything as many have and just do a valve job after ,and if ,the damage occurs.