You should only need to reposition the motor mount posts - measure where they are on yours and put them in the same place.
Normally a bent frame is fairly easy to spot when there's nothing on the frame to hide it. A quick check is to look for cross members being square and then look back to front for any sign of twist. Finally, do a diagonal measurement - from the left rear corner to the front right corner & then this measurement should be the same as a measurement on the other diagonal - right rear to left front. For precise measurements on everything except the motor mount posts, do a search on this site or even on the internet for a YJ frame diagram, there are lots of them out there (many of them show the measurements in millimeters).
$500 is good for a solid frame that has never needed repair.