Harry probably,the best way to answer your question is , long term --NO--,
although there are car covers that ,are are water tight, good fitting, and do help
in keeping your classic ok, in my experiance over 50 years, have done this ,namely
very south Invercargill New Zealand, where i was born, covers dont work.
Here is some of the problems that arise from doing this,damp conditions, mildew,Rodents,
rust and corrosion starting from this enviroment.Probably the worst, is condensation
if you block of the whole car ,so that no air can circulate in and around your car,condensation
gets into all mechanics,ill give you an example.About 1969-76, NewZealanders a huge boating country
that have there boats parked outside, many owners would throw water proof tarpoulins over there boat and there
outboard motors sitting at rear, if long term,many of these engines would seize, I remember Mercury ,sent out a warning about this.
So i would consider you building at least a covered car port , with a quality breathe able cover, on a damp proof ground cover. regards .