Frame out of square

[Sik5chevy]

So I'm down to my frame and it does not look bent but it is 1/2 out of square up front and about 1.25 inches out of square but when you measure over all it's about 1/4 -1/2 inch. I went to mount my gas tank and looked at how crooked the brace was. I can see where prior owner wollered the holes out on the bed braces to fit the brace. I have not taken crossmembers out yet and really did not want to, but some our obviously crooked. If I pop every rivet what hardware do y'all use. I know grade 8 snaps and grade 5 will bend. Any suggestions y'all give me will be appreciated. I hope this is something I can level out and do myself. Long post ..sorry but looking for yalls input that have dealt with this before