Looking at the MWB listings, it appears there is a difference between 74-85 and 86-88 models. In the description for the 74-85 it says "This unit can be used with manual or early-style power windows. Is not compatible with late-style power window regulators used from 86-88". And for the 86-88 it says "Used only with later Bosch-style power window regulators". And you can see they have different tabs at the bottom of the channel/rail:
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There is a slight discrepancy in the years. For the window listings (which include the lower channel/rail) it says 74-84 and 85-88. And the descriptions read like those noted above, but with the different years indicated. So the 84 and 85 year models may be either style? Just check to see what you have, but obviously the change follows the power window lift unit styles.
I'm guessing it wouldn't be too difficult to change the lower channel if needed. I believe they just push into the rubber retaining seal. Naturally you would need to correct channel to match the lift mechanism.