TwistedGA
+1y
Aligning it to the center of the truck should be easy, don't do it from each rail though, both sides should be even from the exact same location (centerline of frame).
Making sure its straight once its centered can be done with the front wheels, measure it as you would for checking wheelbase, from center of hub to center of hub.
This should keep the wheels aligned even if the backhalf isn't square. Once you get into travel, it not being square will bite you in the ass..