ok as far as i know an adaptor plate wont be needed,
and as for fitting, you will see 3 small bolts on top of the original carb undo them and it will pull off leaving the basplate, take that off which is a matter of a few bolts and then finnally the flange which is rubber and black i'd deffo put a new one on, there about 8quid
then the do the reverse with the weber obviouly it'll look slightly different put the proccess is the same, flange base, then carb