I usually give things like that a over-night soak in PB Blaster....
DO NOT use an Impact driver, EVER! It will simply twist the bolt off....and leave half still in the hole.
An old, square shanked screwdriver, one you can "tap" on the handle with a hammer......add a pair of vise grips to the shaft...tap a couple times, try to turn.....tap a couple more times...only this time try to TIGHTEN the bolt....then tap a few more times and then twist the bolt to remove it....Careful..as you may hear a "CRACK" sound as the rusty threads break free.....
Do NOT use visegrips to turn thumbscrews.....you stand a good chance to just twist the head right off. IF you must use the visegrips...keep them in line with the bolt's threads...and NOT at a 90 degree angle....
Another way to soak rusty parts....RBC....Rifle Bore Cleaner.....the full strength stuff the Army uses....