You can't go to a 465 with 208 combo from 700R4-208 and reuse the 208 without replacing the input gear. The 208 has a 27 spline input gear. The 465 is either 10 spline or 32 spline.(32 spline was the factory setup with a 208 behind a 465) As for the crossmember, there are a bunch of different crossmembers with a bunch of different setups, so I can't say on that. I replaced a 700R4 with a 465 and used the same frame mounting holes
I couldn't reuse the crossmember, it wouldn't have bolted to the 465 I had. However my 1989 Suburban had a different adapter on the 465 that was the same as the 700R4 I had replaced once before, so there I could have used the crossmember.