Need frame advice

Hey all!

I am in the process of doing a build on a 1967 Buick Skylark. After removing the body from the chassis I discovered that the drivers side frame rail was pushed back a little more than two inches and as a result the frame was a diamond. I took it to a local frame shop and they were able square it up for me but noted that it was not leveling up, they tell me that the weight of the car will resolve this problem (I am unsure if that is true). I got the chassis home and and placed it on a pair of steel tubes spanned between jack stands and leveled everything up. the front end is completely square and level. On the rear end the drivers side is level with the front end but the passengers side rear is 1 1/8" high at the jack stand. If I set on that corner it levels everything out but it takes a good 100+ of weight to bring it down. The question that I have is how do I fix this? I am planning on adding a rear cross-member, boxing the fame notching 1 1/2" per side for wider tires and adding an Art Morrison style x-brace to the center bay of the frame but before I begin this work I need advice on this situation.

Thanks!

Tom