i really do not see any problems on modifying a car to your taste
it is your car anyway, so do with it what you like, and don't care what anyone else sais.
I also own a very original alfa 90, for 2 years now, not really my sort of car as i've discovered the past 2 years i'm driving it now.
so far i've only lowered it, and put some fat 16" with 225/50/16's under it, all original parts stored anyway
the 90 has been for sale now for over a year, and all those stupid purists, that call themselves alfalovers, had all kind of comment on the car, but not one wanted to buy it, my askingprice is half the insured value, so nothing wrong with that either
but when i told my modifying plans for it 2 years ago, all those clubmembers told me not to, because it would loose its value, are not interested in saving this very original car.
my plan back then involved putting a 1.8 turbocharged engine from a 75 TB in it, and a respray in a modern colour etc.
because i already had the idea that the 90 is not my sort of car, i didn't modify it a lot, and kept it as original as possible
doesn't make a difference at all for the resalevalue
so, just modify the hell out of your car and don't care if it is a rare car
just make it as you want your car to be
or is it just the dutch alfaclubmembers that are so pathetic about originality, but don't want to spend anything on a good car, but prefer to buy a shed for half the price of a good one, and then thousands to get it anywhere roadlegal?
by the way, original also means something no-one else has the guts for to do so