Immovable Mailbox

My house, purchased in 1981, built about 1967, came with a fairly modern metal mailbox, square door and more than ample rectangular space inside.  Over the years the red flag has long since rusted, the name place in adhesive letters long ago has become unreadable and the door hinges have begun to separate.  I bought a really nice plastic one 10-15 years back, stored it in my basement with good intentions, but never completed the replacement.  About six months ago, I figured the US Postal Service would eventually blacklist me, prompting me to bring the new one up from the basement, unpackage it and see what the installation would entail.  Screw holes identical.  All I had to do was remove the rusted bolts from the old one, get new bolts, which I did, and screw on the new one.

Removing rusted bolts does not go easily.  I tried rust remover to no avail.  I tried wedging a hacksaw blade between the screw head or nut to no avail. Tried to drill out the rusted bolts but the bit kept slipping.  I called a handyman who told me he could do it but at a price double my budget.  Have not gotten a bolt cutter, but probably less expensive than a handyman and I'll still have the device whether it succeeds or not.  Last try, then get somebody to do this for me.