[P-51 Mustang]

So I bought some new headers off ebay for my 1998 box with 91k miles on it. I had read a lot of articles on people breaking the bolts off in the head when removing headers, but I thought mine would be easy... I was wrong. I ran the car for a long time before starting the project to try and heat it up a good bit hoping, this would help the bolts come out easier. Once it was hot I soaked the bolts in PB Blaster and started on the left side header first; I got all the bolts off with a ratchet except one that broke. The one at the very front of the car, so I drilled it out and tapped it. The right side didn't go so well, I got the impact hammer and a propane torch and heated each bolt for about 5 mins then tried the impact hammer and I still broke every bolt except 1. I am going to continue this project tomorrow, I just wanted to see what people had to say on the issue. The only way I know to get these out is start off with a small drill bit and start drilling and go up in size until the old bolt is all drilled out then tap the hole with new threads. The only thing is my new threads really are not great... I mean they are OK but I am worried about stripping them out when I go to tighten the new bolts in. Does anyone know how tight the bolts are suppose to be??? Do I crank them down or can I just snug them up?? Is there a better way to do this because I have to do 5 more on the right side, and is there anything else I need to worry about? Any input would be greatly appreciated. Thanks so much!!!