As stated, in an 8.2 '65 rear end, 3.90 gears will not bolt to a 3.23 carrier, which works for 2.93, 3.08, and 3.23 gears. A 3.36-3.55-3.90-4.33 carrier is needed. When I re-geared the 3.55 in my own '65, 3.36 was as low as I could go numerically. Spacers are not a good idea in this already weak rear end. The OP's logic of finding a complete original 3.90 housing with gears intact is the best way to go, and will add value to his car.
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Jeff 
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