Rust free and unbent frames are out there, but they're usually still
attached to high-mileage vehicles driven by gardeners and teenagers.
And rust-free and maybe only slightly bent frames are available from
any decent junkyard in the desert southwest - you can check for bent
before you take it off the truck.

 But be sure to check with your state's DMV /before/ you do the
actual frame swap work, since the chassis serial number for your
vehicle (the number that /really/ counts for legal purposes, not the
plate behind the windshield or the tag on the doorjamb) is stamped on
(you guessed it...) the chassis.

 You may have to get the State to issue you a special ID sticker for
the new chassis, so when a state trooper looks underneath for the
frame serial number he doesn't immediately get suspicious at seeing
the wrong VIN number (from a salvaged vehicle), a hand-stamped number
or a big blank spot (from a reproduction chassis) instead of the
factory-stamped VIN number matching the body tags and the title that
he expects to find there...

 You can avoid a night or two in jail, the impounding of your car,
and a lot of hassles in court to get it back...

      --<< Bruce >>--

Below is the long version of the above, California Law based.  Your
state's laws and procedures probably will differ, but not too much.
Feel free to snip them off if you reply.

http://www.dmv.ca.gov/pubs/vctop/d04/vc10750.htm