What's your bolt spacing across the K-member and across the frame rail?
With the engine in place, it might be tough to measure bolt spacing of the rails, but you might be able to get it for the K-member.
I have 33 5/8" center to center on the main rails, and 28 3/8" for the rear lower K-member inner bolts center-to-center. Those aren't official measurements, but just dimensions on a non-tweaked K-member. One side of the OEM k-member is slotted to allow for some variation in frame rail width, so there will definitel be a +/- on the rails.