custom bell housing won't be cheap, that's for sure.
i'd just get a diesel mondeo mtx75 and whack an atb diff into it.
i run a frs box on my zetec turbo in my xr2, ratios will limit me to 164.5mph due to 15" rims on 195/45 tyres. bigger rims and better top end.
seem to be a very strong box though, shot peened gears as standard and atb already fitted.