blew my u-joint out

That 7 1/4 rear is not so strong to start with so any drilling for hellicoils is not going to help it stay together if you plan on making any tire smoke. If you replace the yoke, that rear/yoke/u-joint will still be a weak link. Its no big deal to blow up 7 1/4 rears but there is always a chance of destroying other parts of your drive line when you eventually take out that rear. Start looking for an 8 3/4 or even an 8 1/4.

I show this picture every time one of these threads come up about drive lines being strong enough. Here is the result of a broken 7290 u-joint (8 3/4 rear). The transmission tailhousing was broken in 2, the tail shaft was bent more than 1/4", yoke bent, and my fancy new heavy duty drive shaft looked like a banana. It was a very costly and time consuming repair for a $30 u-joint failure. The lesson here is not to do anything to weaken that drive line! If anything is going to break loose, make sure its your tires against the pavement.