So long as your pickup is less than 6000 pounds curb weight (including gas and any stuff you didn't remove before lifting it), that won't be a problem.
Assuming that the anchors are properly installed, you should not have a problem removing and reinstalling them. Note that the ones I used were the epoxy anchors; I am not familiar with the other ones (which come standard with the MaxJax). The portability feature was important to me when I bought the unit; however, I have never removed either post since, and do not plan to do so. I carefully chose the locations of the posts to be least intrusive for all of my garage uses; one is near the side of the garage, so it does not create any obstruction. The other is nearer the center of the garage, so I sometimes need to maneuver a motorcycle or garden tractor around it, but it isn't enough of a problem to bother moving the post.
The only other issue I can see for you is post spacing. Pickups are tough, because they are wide, but their frames (where you will likely pick up the vehicle) are relatively narrow. I recommend taking careful measurements on both the pickup and the '57. Possibly you will want a third set of anchors in the floor.