Okay, from the other side, I say go for it. I built my "scooter" almost five years ago entirely from scratch, I've got less than 10K in it and I've turned down offers for almost twice that (I don't know if they were serious offers, but I don't want to sell anyway). To say my bike is useless for "real" riding is, well, your opinion I guess. I built a rigid w/38 degree rake, 8" over forks, a 200 rear tire and it rides real nice. I would rather ride this than the last crotch rocket I had (GSX-R 1100) talk about back pain.
There are so many guys making aftermarket frames, you can really get what you want rigid, soft tail, FXR you name it, you pick the rake the down tube and backbone strech then decide how big you want the rear and what type of front end you want, springer, wide glide, short, streched, the options are really limitless.
I would start from scratch if I were you and get as much as you need in "kit" form. You can get everything from the entire bike to just the driveline or front end in a kit. The beauty of this is that you can get what you want. Get a catalog from J&P cycles and see whats out there, you can spend 40K or less than 10k depending on what you want. But when you are done, it's yours, and if your tastes change, just change what you want to suit.
This winter I will be swapping out my 88" motor for a one that I want to build, maybe 120" or so, I will also be going to a "suicide shift" my first bike was a hand shift and I'm tired of everybody asking if they can take it for a "spin".
I may even go with a springer for nostalgia's sake.
Just for the record, i can't watch that "american chopper" crap, but I really like the "biker build off" series.
Good Luck!
Pete