Richard, you will have to check with the homologation papers of the car to answer your question. You are right abt. the rules for periode "G", - they are difficult to grip ! If you look at en Escort, the use of dogboxes were homologated for period H, I believe together with the BDG alu-engine .
Don't really know if the Elan was ever homologated with a dog-box.....(?!!)
However, in "H" you are a bit more free to claim "periode specification", if you can prove that one particular car used this or that mod historically.
Do believe the dog-box will be a fish hard to land though, at least under FIA App. K rules......
Dag