From my understanding the difference was the transmission. The flat stamped piece was for Turbo 350's the tubular was Turbo 400. They both work with either trans. My crossmember is from a 73 LeMans that had a Turbo 350, my GP has a turbo 400 I used the stamped style. Basically all the 2 door GM Colonnade bodies are the same from the firewall back. Did the same thing on my step bro's 74 Cutlass, removed a 350 Turbo 350 installed a 455 Turbo 400 used the stamped Turbo 350 crossmember just had to move the crossmember back to the rear holes in the frame.
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1975 Grand Prix
1959 F100
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