You need '73 - '74. And make sure it is not from either an El Camino/Sprint and or possibly a wagon. Their frame rails were different (closer together) making the crossmember shorter. At least that is what recall from the '68 - '72 generation. You might try the third gen site (www.g3gm.com). There are likely aftermarket solutions - GForce, IceMan, etc. Wish I could help, but those cars have dried up in the U pull its..
I bought mine from Sumiit and it fit perfectly. https://www.summitracing.com/parts/sum-770322/overview/year/1976/make/buick/model/regal and https://www.summitracing.com/parts/sum-770323/overview/year/1973/make/buick/model/regal. These are made for Summit by G Force. https://www.crossmembers.com/products/?find_make=buick-model&find_model=regal&find_year=1973