Rear Versus Front Tire Widths

So, as I am narrowing down new wheels and tires for my 67 Barracuda, the thought occurs to me....
Although I am used to seeing all 4 tires the same width on passenger cars, that does not seem to be what I see on Classic cars.
In order to get my wheels and tires back into the wheel wells, I intend to go with narrower tires.
Currently, I have 255s on the rear; and 205s on the front.

So, the question it begs is.....
Can I go with 225s all the way around?
Why fatter tires on the rear?
Why so narrow on my fronts - 205?
What kind of car?