You could heat it with gas but I've found gas won't get hot enough to work effectively.
Maybe try another slightly smaller size socket ( even try metric ) and see if you can knock it on to the head of the bolt so its tight and give it some twist to see if it slips or holds on the bolt head.
From memory those two bolts are 7/16th head ( or maybe 3/8th

Or if the head is really rounded try knocking on a 3/8th or 10 mm socket.The vibrations from even knocking on the sockets can help to loosen the bolt/nut's hold to each other.
There are a number of specialist tools available to buy that are designed to remove rounded off bolts/nuts/studs etc that may help but you would have to go to a tool specialist to get them.
Or if everything else fails you could try and grind off the bolt head and drill out the bolt thread and put a new bolt in ( if the thread is still OK in the nut that is ) or use a new nut and bolt.
An air driven ( or electric ) die grinder is the best tool to use for the grinding down of the bolt head if you have one otherwise you will have to try and see if an angle grinder can be used.
Cover the interior of the car before grinding as upholstery doesn't like grinding sparks!!
Or as Trev has suggested, cut extend and weld the pedal arm you want to lengthen in its current position without removing the pedal assembly but from memory, its not in an easy to get at position where it is and could be a pain in the arse to do it that way but if pedal assembly can't be removed then thats the only other way to extend the arm.