Tax exemption?

I don't want to open a can of worms but...
How much of a (pre 73?) Landy needs to be original for it to be tax exempt?

I haven't got the specifics to hand but DVLA operates a points system and you need to retain 8 or 10 points. IIRC the chassis is worth 5 points and the suspension 2points with the engine been worth 3points. But I may have got them wrong. As I said do a search for it. It's been covered hundreds of times.