are these g-boxes directly interchangeable. will my suby adaptor kit
for 1600t2 fit an 1800 t2 g-box

i'm 99% (maybe a touch higher) sure of yep.

The 1600 - 1700 - 1800 all use the same bell housing and off set on the flywheel.
The 200mm clutch up to 215mm clutch all use the same ring gear at the same off set and the same starter motor.
Even the early 1600 t2 bell housings with out the rabbit ears are the same internal size as the later,
Only the 2L or 228 mm clutch is different, then we have a problenm with interchanging parts.

Hi

The 1600 uses a shorter main shaft than the 1800, they are easy to swap over.

1302Steve