2A replacement chassis

I am the lucky (Un!) owner of a 2A with a rotten chassis. It was a range vehicle for the British Army in Canada and is fitted with loads of radio gear mounts and battery boxes etc. More switches then you would ever belive possible and is in need of a rebuild. I have been reading back posts about keeping tax free status (which this one has) when changing components such as the chassis.
Despite all the information on the forum I am still confused as to the points system for a chassis replacement.
Question, if I replace the chassis with a brand new galv chassis does it mean 5 points knocked off or no poins as it's a like for like replacement?
Second question, how much of the landy can I keep in one bit to change the chassis? I have a JCB to lift off what I can without unbolting it all, what's possible and what's not!
Third question, has anybody got any service information for the huge Sepson winch fitted to the front of this 2A! It's the 24 volt 2.5 ton version.

As far as i understand change means a modification that was not available at time of manufacture, so a new chassis is ok. Any new part that is a pattern of the original is ok.

There are a possible total of 14 points for tax exemption. You have to retain 8 points to continue claiming tax exemption (Historic Status).

1 Point for the engine.
5 Points for the chassis.
2 Points for the transmission.
2 Points for the steering assembly.
2 Points for the suspension
2 Points for the axles.
= 14 Points in total.

An addition to the vehicle does not take away pionts, so power stearing does, but the power stearing bolt on kit that is an addition to the vehicle should be ok, but i am less sure of this bit. Disc brakes replace the original etc etc