3/4 T brackets are different than 1/2 T. Did they give you the correct ones? Or trying to use 3/4 on a 1/2 frame? What are you going by to say 3/4 T truck? 8 lug, VIN, etc? I assume the 6" mismatch is width (side to side), not length (front to back). Of course, if they gave you the next generation brackets 55 and up, that could be trouble. You have two sets of brackets and they fit the same? , which is: they both don't fit? Have you looked at another 3/4 T truck to compare/measure your brackets and bumper? I'm not questioning your ability to know if you have a 3/4 T and the 3/4 T parts, but to trouble shoot, you have to question/revisit every "thought" you have. Obviously something you thought was right, is wrong. Wrong frame, wrong brackets, wrong bumper, assembled wrong. Brackets (and even bumpers) can be assembled flipped, rotated and wrong side.