Re: Fast Eddie here...
Eddie, (you're the second fast eddie we have here),
Welcome to the forum and glad you've joined us. Your old school cruiser sounds like an interesting project and you have chosen a good solid platform for it. There is no problem with using the 3 speed hub and coaster brake but many of us stress the importance of adding a brake to the front wheel. I can see your point about welding on a classic fork like that so if that is not an option then consider adding a 'V' brake to the rear. Another option is a disc or drum brake that can be adapted to the front wheel.
You'll probably run into some problems when mounting the front of the engine. Most of the cruiser style frames require a custom mount at that point unless you want to consider drilling a hole in the down tube to use the kit supplied mount. An option that I'm sure you, and many of us frown on. Keep us informed and feel free to ask if any questions come up during the build. Oh, and don't worry about backfires and being launched. It just doesn't happen.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"