What I always look at is the shape/font of the WD45 radiator decal.
The ones sold here are an odd font, that doesn't resemble that in the parts book at all.
I think http://valubilt.com/ sells the right one.
That said, one of our tractors we used whatever type it is that comes with the "film" all the way around the Allis-Chalmers part on the hood, and though it is holding up well, that clear film shows quite a bit (Dirt has even gotten cough around the edge, ie its a working tractor.)
Last set we bought, but have yet to put on, but it is the type that all the letters are separate. Not sure yet how easy they'll be to put on or not. Those I bought on eBay, as a WD45 Diesel set, and will just throw away the Diesel part.
All said, I go for the cheapest I can find, so long as the font/shape is correct, and for what I've seen, the WD45 for the grill is what often is wrong.
Finally, chances are the set that we have yet to use will be the last WD45 set we buy regardless of how well we like them, as I wanted this tractor to have the traction boost decal for the battery box, but any others, I'll be happy just hand painting "Allis- Chalmers" and WD/WD45 on. But I know that's not for every one, as its not exactly original, nor is everyone good enough with a paint brush.
Hope that helps.