Dave,
Cheers for that, I thought you were in the US. They don't have as many 80's and 90's FIATs around (well, more precisely, none
So just beware of steel wheel bolts - they will be too short and may even have a different taper angle. Pick some bolts off an alloy-wheeled car (such as an Uno Turbo) and you should be fine.
My local tyre shop has provided me with two sets of new wheelbolts for past projects, at a reasonable price (equivalent of about 25 quid/set of 16). Since they're chrome-plated, they certainly smarten things up. Also they have a flange at the base of the hex-head, which prevents the wheelbrace from going on too far and digging into the wheel.
As a general engineering rule, the bolt should go into the hub about as far as the diameter of the bolt - make sense? That means, realistically, at least seven complete turns once the bolt is 'started' (and preferably a couple more).
On the other hand, if the bolt is too long, it may foul the suspension knuckle (and then be impossible to remove once it's 'smeared') so take a little care to make sure the bolts are the right length.
-Alex