I've modified a driveshaft before and it wasn't rocket science. Last time it was the front shaft on a four-wheel-drive and it worked fine for several years. I'd had a constant velocity u-joint fail and rather than replace it, I put in a longer shaft that I made up myself.
Now days I'd hesitate to use a home built drive shaft in a daily driver or high stress application, but if you are a good welder and know how to balance the shaft I don't see a reason not to go ahead.
