The chevy transmissions have a different bolt pattern from the engine to transmission. This includes manual trannies.
The Buicks, Oldsmobiles and Pontiacs have the same engine to transmission bolt pattern. Hence the reference to BOP.
You can use a chevy tranny wit the 403 olds engine in your pontiac, BUT you must use an adaptor plate as was previously mentioned.
BTW, TH350's came in 6", 9" and 12" tail shaft arangements. Hence the reference to short, medium and long tailshafts.