Celica box is very strong, they came out of not only Celica but T18 corolla and maybe some corona in the 70s to mid 80s.
Instead of a Dellow conversion, there's another plan of attack is a billet aluminium spacer between the box and belhousing, which keeps the escort belhousing, this is easy cheaper than a Dellow system.
More alternative is to find a Mazda rx8 6sp and get an adaptor plate(or make one) from UK. They use Escort clutch spline and same tail shaft but Mazda has a slightly bigger diameter.
Nobody uses BMW getrag boxes here in Escorts, cheap, strong, compact, yanks love them ... I'm deciding to get one soon for my mk1 Escort, not sure on engine going in it yet, zetec , been there done that, duratec, yeah still a good swap . K series Honda, might save that for my 4x4 ute.
Or.... Don't laugh now... A Suzuki Vitara/Swift Sport 1.4 turbo motor, these are potent little units and light weight, economy is a ridiculous 5.2/100kms or less on hwy !!