The frame rails run parallel all the way from where they terminate in the rear to a point just in front of the rear mounting eye for the front springs. At that point they start to make a gradual curve inward. The inward curve ends just forward of the front axle and they begin to run parallel again. They stay parallel for the rest of their length at the front terminations:
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So, if the x-member is to be added in the area where the rails stop running parallel and taper inward, its not possible to give you an accurate rail to rail inside dimension. But the good news is, my 86 K30 is currently down to just the frame and cab. I would be happy to get that rail to rail dimension for you - but you will need to provide an accurate description of the intended location. It doesn't matter what you use as a reference point. You can give the location as a distance from any easily accessible point on the frame or a component attached to it. For example: the front or rear spring mounting eye, the existing front cross-member, the motor (or transmission) mount support, etc.
You can use the dwg an parts list below to help decribe where you measured from:
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Let me know.
On the other hand, if the modification is to be done either before the start of the inward curve - or after it ends - no longitudinal distance is needed.
