V6 conversion = Engineer's Report, discs, probably a chassis kit, lane change tests at AIR ($$$s)
Lot's of hassle and dollars, but if you are keen it is not impossible.
Up to a stock 202 you are OK with just Regency Inspections, some smaller reds you can keep drums all around, front discs are required for 173 (maybe 186) and bigger. Only if you refit a grey will you escape a trip to Regency, you will still need to inform them of engine number changes.
Drop by Regency and pickup the leaflet for Early Holden engineering, it lists most of the common changes that are allowed and the rules for them.
After that it really is pot-luck, if you don't like the answer you get one day call back in a day or two and they'll give you a different one. I think the system is setup to discourage any sort of modifications at all, plus keep in mind a lot of the guys out there are not mechanically or engineer minded
Before you buy a motor, get the number checked with the Police
I learnt the hard way with my car being impounded for a number of weeks because it was listed as stolen 15 years earlier
My advice, stick to a HR front and rear end with a red six to minimise the hassle and cost.