Re: Stunt bike build
By the time you are done getting a bicycle frame to withstand what you will put it through you will have a motorcycle. Not only that you said in another post that you were not going to use the pedals.
An XR80 only weighs around 150LBS. With some after market products and stripping it down you could loose a lot of that weight.
I am not sure what type of "stunts" you plan to do, but maybe you should look into a 80cc trails bike
In the long run it might be cheaper and for sure it is much safer.