'68 383 barracuda fastback - what to look for?

Look for rust or rust repairs in the bottom rear quarters, lower rear corners of the fenders, the area on the rocker panels just below the door jambs, bottom corners of the back glass, and of course the floors in the cabin and trunk. Look for things that would be found on a factory big block car like; large capacity radiator, large brakes, adequate torsion bars, 8.75" rear end, adequate leaf springs, etc.. It sounds like the torsion bars may be set too low or something, I would have it checked immediately if you buy the car. I would check to see if the steering linkage hits anything especially since it has headers on it. Look to see if the idler and pitman arm clear everything, especially the oil pan. If the conversion was done with the proper parts like those made by Schumacher (engine-swaps.com) then there should be few or no problems. Good luck!!!