I don't know if you are going to get enough heat with the propane tank or not. There are bottles of Map? gas that heat up to a higher temp. I have used the oxy/acetyne to get the bolt hot quickly so that it will "stretch" the threads and loosen them with it cools.
You might have to resort to drilling and tapping them out. EZ outs sometimes work but those bolts are 5/16" so the EZ out would be a little bigger than 1/4" and might be succeptable for breaking. The other thing is if you have been heating the bolts and then spraying them with the penetrating oil to cool them, the bolts can become tempered and extremely difficult to drill.
good luck,
Bill M