How hard/how expensive is it to put a/c into a non a/c car? I have an E body, 318 engine.

several companies have complete kits. classic auto air and vintage air are the 2 big ones.
buchillon(sp) is another that makes the underhood parts.

ebodies are popular enough no cutting is required.

should run you around 1200-1500 depending on a few things. but that is soup to nuts, nothing else to nickle and dime you.
kits start at about 1k.

What do guys do when there cars have no dash vents? Just at your feet and defrost?

no, they make vents that bolt to the underside of the dash, to point it up to your face or body.
or others have changed to a/c dashes. just depends on how much you want to spend.
I even recall seeing one where the guy cut out the dash to mount vents.

Thanks for the tips!