i'm not sure about older freewheels being smaller.. i know they are smaller width-wise, but i think the part that screws onto the freewheel is the same size regardless of year.
when you switch to a single speed freewheel, you will need to respace and redish your wheel to make sure that the cog is lined up with your front chainring. re-dishing an old wheel can be a pain in the butt if you have any stuck spokes so make sure to lube them all before trying to turn any of them