Fine, but don't go bitching when you get the vehicle crushed, plus a fine along with a conviction for fraud. It's nothing to do with being 'PC', more a question of whether you want to do it properly or not. Guess who the DVLA are targetting????
One small point. A quite important one..... If you modify the chassis by changing the suspension you will need a SVA test done. There is also a possibility that they will Q plate it as well. It should be picked up at the next MOT, so not worth it.
There are also minor differences regarding the brake line brackets. Basically you can fit the series 3 chassis underneath a series 2/2a, but you will need to do some minor fettling.
Be aware that 'technically' it isn't a direct replacement, therefore the vehicle could lose its historic status. But you'd need a very sharp eyed DVLA official to spot it.