The 63A was indeed a one year only item, B210 GX in 1978. The 63A is an L series transmission, with a special sand cast bell housing or front section. You can see this readily if you compare the sand cast front, with the much cheaper to manufacture die cast section at the rear. When it's pointed out, you will be able to spot one from 25 feet away while walking through the bone yard. Normally, OEM's stay away from sand casting if possible due to the higher cost versus die casting. The 63A was a limited production run, so it wasn't worth the expense to tool up for die casting the part. The 63A uses an L series output yoke, and the clutch disk will have an L series spline. Both input and output splines are the same. I'm not sure that an L disk will fit though, being it is probably going to be larger in diameter. Your best bet for a clutch disk is to get your old one rebuilt. The distance from the in/out centerline to the bottom of the case will be larger on the 63A, as the spacing between the main and countershafts is larger, meaning the gears are larger in diameter although it is going to be negligible. The 60 or 63 refers to the distance in mm between the mainshaft and countershaft. The bottom of the 63A might hang a couple mm lower, maybe 6-8 mm max?

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BTW, it's entirely possible that the 63A sand cast bellhousing will fit the standard shift pattern L transmission. I have not investigated that.
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