Re: 91 GMC Crossmember
The GMT400 is the platform designation for the '88-'98 full size trucks. The '87-'91 R/V series is basically the same as the '73-'86 trucks. The '88+ trucks have a much wider frame than the '63-'86 ('91) trucks. I would measure from the mounting surface of your trans mount to the top flange of your frame and see what it comes out to be and go from there.
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