TBH it is what they advocated in the Champion book, and it is mentioned in the roadster.
It all depends how confident you are with your welding, and if you can get the right materials, although I am assuming the propshaft needs to be shortened (no donor)
You could always take it somewhere and have it checked afterwards, and maybe balanced.
If you are really worried, but intent on doing it, why not fit a safety loop? The yanks do it quite a lot on drag cars
http://home.fuse.net/fastmonte/DSSL4Gbodies.htm
I just googled "prop shaft safety loop" and that was the first thing to come up.
I'm sure someone will be along soon to say "its a dumb/crazy/stupid idea" but then again so is building a car from scratch
Sean