Need some standardization here. Are we using GT350H, GT350 Hertz, or both?

You forgot about here, under Shelby:
http://www.imcdb.org/vehicles.php?make=Shelby&model=GT+350+H&modelMatch=1&modelInclModel=- on

So indeed we should really standardize that

Also the other 350s & 500s. We have:
Ford Mustang Shelby GT350 and Shelby GT 350.
Ford Mustang Shelby GT500 and Shelby GT 500.
Ford Mustang Shelby GT500 KR and Shelby GT 500 KR.

And in a parallel duplication lots of AC Cobras and Shelby Cobras, with the occasional same car (from chassis no.) occuring on each side of the fence. Should we remove Shelby as a maker name for all Mustangs and Cobras, but have it in the model name as appropriate or if necessary agreed format? We may still need to recognise Shelby as a maker name for other
model/types, so it would not automatically disappear.

And before these discussions get too bogged down, sw68's original question still stands and can probably be decided fairly quickly. Not my specialist area at all, but I understood 350H was the official title at the time?

Indeed first we have to clean the Hertz one, we can see for the others later

Assuming Wikipedia is correct, there never was an official Shelby Cobra roadster so they should all have AC as the make.

I got tired of finding the Shelby Mustangs under both Ford and Shelby, so I researched them to try and make sense of them. According to Wikipedia, from 1965-68 these were made in Shelby's own plants, and the 1969-70 models were made in the Ford plant in Los Angeles. The name "Mustang" only appears on promo material for early-1965 only, and never on the cars. Late-1965 and the rest of the years did not have "Mustang" on the promo material. Here's what I did:

I renamed all the 1965-68 models as brand-Shelby, model names-GT ### XX (with spaces)

For the 1969-70 models, brand-Ford, model name-Shelby GT ### xx (previously the Fords were GT###xx-without spaces; I took out the "Mustang")

I left the modern ones alone for now.

There are a couple changes we could make:

For 1965, change model name to Mustang GT ### xx, under Shelby brand

The 1968 promo material lists "Cobra" as part of the name. It's not listed in the other years that I found.

Include the 1969-70 models under the Shelby brand. I listed them as Fords since Shelby didn't assemble them.

Please let me know if we should name them some other way. Based on what I could find, I thought this was the best way to make it easier for everyone.

Thanks. I didn't notice there were many wrongly listed there.
For modern ones they seem more listed in a more coherent way, under "Ford Shelby" make (there was just one as Ford Mustang Shelby that I changed)