As for sealing the top tube. The magic ingredients are JB Weld and tank sealer. It is impolite to post links to other forums but HBLocal has an excellent post on the other forum about accomplishing this.
As for the coaster brake and the Shift Kit. Not going to happen. It just won't work with a front freewheel. It is sort of like asking how to hold water in a sieve. By the time you modify the sieve to hold the water it doesn't work as a sieve anymore. You may be able to get a disc brake to work however. There was an aftermarket disc brake mount that I used on my bike. I don't have the link handy but I did post about it in the thead in the pictures gallery about my bike. I also recall a post by Mabman that had some links to very nice disc mounts.
Dug out the link for the disc brake mounting bracket I used: Disc Brake Bracket Billet - Choppers U.S., LLC
Nice job on the polished covers!