Okay, pay attention.
1) The ported barrel is not exclusive to the Glock 18. In fact, Glock discontinued the 18C because when they made the 18 full auto, it balanced so well cyclically that compensating the barrel actually made it harder to shoot.
But a compensated barrel, even fully functional, is perfectly legal and the choice of competition shooters everywhere.
2) I'm not getting "the lowered area between the ejection port and the rear sight" -- do you mean the top of the slide? I don't see a difference between the 18 and the 17 from this picture:
[URL=http://media.photobucket.com/user/BADMAN2006/media/g18e.jpg.html][IMG]http://i48.photobucket.com/albums/f222/BADMAN2006/g18e.jpg[/IMG][/URL]
3) How are you planning on "removing" those grooves? On tenifer coated steel, this will be quite a trick. Probably more trouble than it's worth.
What you are proposing is cosmetic and technically legal, but ask a guy about when he put a "fake" selector on his M1A to fill in the selector hole on his M14 Stock. The ATF was not happy with him. Granted, this was 20 years ago, but the ATF still has no sense of humor about things...might want to check with them, as well, not us. We're not lawyers.
It sounds like a great mock-up. Just make sure your parts are durable, match the finish, and are absolutely NOT operable under ANY circumstances.
Storm Lake makes compensated barrels, as does Lone Wolf. Those would be my two primary choices.