I believe there are two Offy options, one for sideways, one for setback/forward. Could you have bought the wrong ones?
I believe the Offenhauser #5880 provides the pair of spacers which offset each carburetor 1/2"; whereas the #5832 provides the pair of spacers intended to rotate the carburetors 90 degrees. Unfortunately, the later is intended to be mounted to manifolds which present the 5-5/8" x 4-1/4" bolt Carter AFB & Quadrajet pattern, adapting the 5-5/8" x 5-1/4" Holley pattern for the carburetor mounting.
Hence, why one may find that the best way is to do a little custom welding & machine work to present that which is needed.
Oh,............I don't know what size of a welder you may have acquired but when welding aluminum, figure you'll need twice the amps as that of steel, and I would guess that for this job anything less than say 200amp. capacity just isn't going to be enough for a good result.
Scott.