You don't need to replace the transmission if you have an SM-420 or an SM-465. Just change the bellhousing. I'd freshen up the tranny though, so that it has a chance with real torque.
The motor mount position and type is exactly the same as a small block Chevy, so the same kits will fit a BBC. It is a wider engine though, so I'm not sure about headers. You might be able to do custom headers that come out over the frame rails and into the wheel wells.
You should have adequate room under your hood for that motor though. Make sure to upgrade your radiator. Your front spring rates will be fine though, since a 235 weighs about the same as an all-iron big block Chevy.
