Couple of things:
Old Aris 8 freewheels have the correct 2.0 spacing for Campy. Newer 8 speed freewheels likely have 1.8mm spacing.
The amount and location of the respacing you do is up to you, and it doesn't have to be 2mm per side. If you can get away with 1mm on the DS and still keep the chain clear of the chainstay, then do that and put the other 3mm on the NDS. That will minimize the tendency to damage the longer DS unsupported axle and will improve dish.
There is a fair amount of space to play with - I have several freehubs with added 1mm spacers under the cassettes and there is still room for the chain in high gear.