bogey to single

evening all a mate is thiking of converting his walking beam bogey in his acco to a single drive airbag rear A does he need a engineers certificate? B can he derate g v m for cheaper reg?C what would be his g c m? by the way based in victoria cheers whale

g/day john i did my isuzu about four years ago in nsw had to engineer changed the gvm to 16 even tho axle limits is 15 and gcm stayed as it was,rego from 4odd thousand down to 12 0r 13hundred cheers col.