This is one of those weird applications where you do not need to insulate the TO-3 from the heatsink because it is common collector. You should insulate the heatsink itself, though, because it will be charged to +40V. It's a good idea to heatsink the pre-driver because those old 3055's got lousy gain, so the BD243C is cranking out a lot of current. That 10 amp diode is not critical, any normal recovery rectifier rated for at least 10A and at least 50V will sub for it.