Thanks for the ideas Rick. I also noticed that both frame horns were biased to the passenger side in my jig so that verifies to me that the crossmember is sitting slightly crooked. Since I marked the centerline of the frame based on the radiator support holes and not the midpoint of the frame horns, I think it's still accurate. In order to accurately hold the frame it looks like I'm going to have to bolt the jig to the frame in the rear, which means I have to remove the bumper and move or remove the tailpipes that are right smack in the way. I was hoping to avoid that.

We'll be cutting tomorrow, so I'll work some more on setting up the jig today. All in all, I'm happy with how it's working out.