You do NOT want to **** those threads in the frame. It Sounds like you've done enough damage with the torch. Put that crap away and let's do this the patient way.
Step one: take an easy out or broken bolt extractor and throw it in the fuckin garbage where it belongs.
Repeat until you're out of those ******* things and then stab yourself in the leg with a pocket knife next time you think about buying one.
Step two, drill a 1/8 hole in the SIDE of the frame beside the bolt, so you can spray penetrant on the threads exposed inside the frame rail.
Step three: heat that broken bolt with a propane torch and soak in the thickest penetrating oil you can find. Hit the amputated end and the new side hole.
Step four: start drilling. Center it up as best you can and drill it as plumb to the bolt centerline as you can. It's not a blind hole so you don't care about using left hand bits. If it comes loose and spins up into the frame, count your blessings and move on. If not, drill until you break through.
Odds are, you'll get into a fairly large drill bit that's going to get close to threads on one side, but when you start getting close it'll probably come loose. If it gets to this point, try to start removing material in the thick portion of it so you're not trying to drill something that's loose in the hole while it's dancing around messing up threads.
A carbide Burr on a Dremel, some safety glasses and ear plugs, and some patience will win this battle.
Then drill that 1/8 hole for the other three bolts and spray em BEFORE you try to remove em.