Personally I think the truck looks fine. Having seen photos and models of actual LG3000's, I can say they did the best they could. It's at least as close enough to the genuine article as the faux Tigers in Kelly's Heroes. Which is to say...enough that you can tell what it's meant to be.

I guess a good replica is in the eye of the beholder.

The main indicator it is/was a GMC CCKW seems to be the wheels and the body length, to hear the experts talk about it (and to look at photos of the genuine article), but honestly, are people coming to the site going to know or care about details like that?

It is indeed a problem... several problems actually.
If we list the vehicles as made for movie or their real base, people who search it will never find it, e.g. the Ferrari & Lamborghini of http://www.imcdb.org/movie.php?id=106309
(and they are not "made for movie" anyway, just replica/kits)
On the other hand, some of these "replicas" are so bad that they do not look enough like the n actual vehicle. Like the truck of Raiders of the Lost Ark, it is strange to list it as Mercedes IMHO
In a way that problem is linked to CGI & scale models, which are also approximate replicas of vehicles.

Looking over some of the films here I've noticed that the stance on replica vs. made-for-movie is a little loose. Like, why are some vehicles listed as replicas, while others, equally identifiable, get listed as made-for-movie? See Raiders' 'Mercedes 320 Replica' vs. the same film's 'Made-for-Movie Mercedes G5.' What's the difference? Should we list the 320 as made-for-movie, or the G5 (and the LG3000) a replica?

(I know most people are probably tired of hearing about Raiders' fake German vehicles but it was only example that sprang to my mind.)

And if we're listing replica vehicles as what they're supposed to be, why are all of the faux Abramses and Tigers listed as Centurions and T-34s?

When we are sure that it is a replica (destroyed, visible differences, reliable source, etc.) we can mention it. Else we can just list it under what it is supposed to be I guess.
For AC Cobra it is more difficult since many of the replicas have an actual make/model name, some are properly listed on the site, but most are just listed as AC until someone indentified which replica it is.

I don't know if this has been discussed previously, but I am wondering what the majority feel is a suitable way of identifying certain vehicles of which there are a lot of replicas or special models which may only be clones.
In my opinion if it looks like an apple we should call it an apple, unless there is some way of knowing for sure it is an orange made to look like an apple.
As an example the Ferrari California in Ferris Bueller's Day Off http://www.imcdb.org/vehicle_2864-Ferrari-250-GT-Spyder-California-1961.html?PHPSESSID=1104fe56bbf9006d1ecee2f3eae29644 was a replica (no one in the right mind would destroy a genuine one) and it is commonly recognised as being a replica, so therefore it is listed as a replica.
What about AC Cobras etc. If they are genuine or replicas is sometimes not able to be determined, except maybe if they are destroyed it can be assumed they are fakes.
Another case is a limted built/high performance version of a particular car. As an example a Chrysler Valiant Charger R/T http://www.imcdb.org/vehicle_186449-Chrysler-Valiant-Charger-VH-1971.html appeared in The FJ Holden. I know this car was extremely rare, and nowadays is commonly cloned, due to the limited number built, and the high prices for genuine cars. However in 1977 when the movie was made they were vitually worthless and unlikely to have been a fake so it can be assumed it is genuine. If it were to appear in a modern film I would assume the opposite, that it is a clone.
Should there be any guidelines for these types of cars?
Should we assume they are genuine unless there is evidence otherwise?
Or should each case be taken individually?