Any BOP tranny will require you to swap out the output shaft from a Chevy 4x4 TH400. You might as well buy a rebuildable core and have it rebuilt while it is torn down. I have a core TH400 with the BOP pattern I would sell for $50.
Other items you will need:
Chevy TH400 4x4 output shaft for NP203
Chevy TH400 to NP203 adapter
32 spline NP203 input (does not require any modification of your case, just swap out the 27 spline input you currently have)