Couple of tidbits of info I know:

1 - The license plate -- "THX 138" -- was the name of George Lucas's first film project in college I believe. It turns up later in a few more movies, and of course, "THX Sound" that Lucas Arts created for surround sound in movie theaters. Jeff Beck has a pretty decent instrumental that took its name from the coupe's tag.

2 - Falfa's (Harrison Ford's) black ‘55 Chevy is the same ‘55 used in Two Lane Blacktop. It was left lying around the studio prop department and was refurb'd for Graffiti. Both cars (the ‘32 and ‘55) were left to rot pretty much -- no covers, left out in the weather, etc., through most of the ‘70s. The same guy owns them both now, and has them restored to as-appeared perfect.

3 - Flatlander's Hot Rods sells the original Milner's Coupe with the drive in in the background poster. You can get their name and address out of any rod magazine. They sell a very close repop ‘32 modeled after this coupe.

4 - I am sure you can get the repop movie poster on eBay for cheap. I bought an oversized one, like 40x70'' for $10 on there. Very nice piece.

Again with the useless knowledge,