Overdrive repair may be as straight forward as fixing the solenoid.
You don't say what model O/D it is n(A-J)
If you get into gearbox changes you would need to likely change the prop as well, tunnel mods, matching gear ratios & diff etc.
Not forgetting the OD ratios(%) should be matched to the engine & diff. The J is available in 25% & 27% ratio of input to output speed.
I believe that a 27% with 4.2 diff is recommended for the Bristols with a 100 engine but am no expert.
You seem to be starting from a setup that's all there & hopefully correct except the O/D needs fixing.
Geoff Dowdle in Australia is in the process of making an adaptor plate conversion for fitting a ford T9 5 speed box to the Bristol 400.
I am fitting a new overdrive (J) to my Bristol 400 and it came from :
http://www.overdrive-repairs.co.uk/ they may be able to offer a exchange/replacement.
Graham