Do not use a inpact, you WILL break the bolts
Spray PB blaster, let it soak in, spray again, let it soak in, do this 3 or 4 times.
Hit the head of the bolt with about a 3 lb hammer, do this 3 or 4 times, spray again between hits (hard hits).
Try to move the bolt with a wrench, NOT a breaker bar, tap the wrench with the hammer if needed.
If no movement, start over, the trick is to get the PB blaster into the threads, when you reinstall the bolt USE anti-seize.
I had to change my tank on my 73 m819, I did it WITHOUT breaking the bolt.