If it's an original car I would loath to modify it. I used to own Alan Manns 107E Prefect (reg 33KKR) when it was an unmodified, totally original, never welded, one owner, 42,000 miler car. I personally wouldn't have modified it, but now it is I don't feel any anger or bitterness, because he's done a quality job.
Had a conversation with my work colleague this morning about me putting a cage into the Anglia I am restoring. He said 'Won't the purists complain?'. I said 'Yeah probably but I've rescued this car from the scrap yard so I can do what I want with it'.
So in a nutshell, if it's mostly original, I would keep it as original as possible. If it was a shell or have done loads of welding to it, modify away!