If I am fitting a two Litre engine in place of a 1600 do I need to change the gearbox too....?????

dozza do you mean a type 4 engine...like from a kombi?
have a look at http://www.tunacan.net/t4/howto/starting/ 
steps 6,7,8

no you dont have to i ran the stock box with a 2L t4 but i would recommend changing to a single side plate box or upgrading to a heavy duty side plate on the left hand side.

I mean a standard 2L kombi engine from say a 1976 instead of a 1600??

yeah thats what i run in chaos. it only requires a 1800 flywheel, a different input bush and and upright conversion kit in a bug