they shouldnotbe hard to modify as passat estate tanks are 70l and they can be replacedd to fit 100l. i think you can fit the 70l passat tank on the golf estate. i may be wronh though.
update- there is an air reserve in every tank. the little plastic thingy above your tank opening will let you decompress it and then you can fill ir up above the max. my friend fills his b4 estate with a 70l tank up to 90l and hasn't had any problems. the diesels have larger tanks and fill up past 100l. good to know if you got to go far so you can fuel up once.
I don't know if this damages the fuel syste
m or not. people do do this.

update to update
you can damage you EVAP by overfilling it. this is a evaporative purge regulator valve. this thing is close to the airbox on my car and i always wondered what it was...now I know. Live and learn.

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