GMC vs. Chevy manual transmissions

[69GMCLonghorn]

I need some advice: I just bought a Gear Vendors to "bolt up" to my SM-465 transmission... or so I thought. I dug into things and discovered that GMC did not use the SM465 transmission; they used the New Process NP435, which was, for the most part, used in Fords and a few Dodges for about 20 years. Gear Vendors had never heard of that trans being in a GM product, and therefore COULDN'T SELL ME THE CORRECT ADAPTOR. As we speak, the unit is being boxed up and being sent back.

Has anyone else but me experienced this? Or did someone Mickey Mouse my truck prior to me owning it? Any info would be great.