loading cars in a 7' can be a little tricky. if your corvair has only 5" total of extra width, 2.5"/side - that is going to be REALLY tight. odds are you'll manage to prang it given enough load/unload cycles. with such little clearance, you'll just about have to get a winch to load/unload. there just won't be enough room to get out of the car once it's in the trailer, even through the window.
not that its a major concern (probably), but resale value on a 7' is generally much less than 8.5'.
as far as towing goes, the wider stance will probably sway less, although trailer's propensity for this trait seems to vary between mfr's. so one mfr's 7' might sway less than another's 8.5', so tough to generalize. also, anti-sway systems like reese's dual cam or the old school friction device should take care of it.
ahm