A shift kit can give you a much broader speedrange, tho. If you want to cruise at high speed but also have excellent acceleration off the line, a shift kit's the way to go, if you have the means and skill.
OP: I can try to help
1) Like biknut says, if you have an NT, keep it. It's a great all-around carb and easy to set up right.
2) Get SBP's universal front mount. It's solid. Get their HD bolt kit, too. It's easier to get the engine mounted flat and solid with their front mount.
3) There are some threads here about making your own frame, use the search box above the forum header! Culvercityclassics has a thread with an easy frame jig in it, too.
4) Yes you can, many bicycles, specifically multispeed cruisers with rear derailleurs, are set up that way. That really doesn't have anything to do with your frame, as any bicycle with multiple chainrings can be switched to single chainring. If you were to go with a SBP kit, you'd be running a dual chainring system anyway. Before you ask, yes it's easy to convert either way
The shift kit itself tho, takes some serious aptitude and incurs extra maintenance and other teething penalties