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Wire brushing and grinding is the pits, extremely time consuming and very easy to take off too much metal. Truck frames have rivets which can be hidden by flaking rust, and you don't want to behead those, even partially.
I'd rent a sandblaster, and have at it. Wear leather gloves and goggles of course, and have a shopvac handy for afterwards.
I'm about to do this myself, once my sandblaster arrives. I bought a 10 gallon unit on ebay for $60 plus shipping. I was going to rent one, but the two rental places have larger, 20 gallon units for $100 a day. I figured might as well just buy one.