Can't help with design or purpose on all 3, but I have used all 3 in Mustangs and can tell you a little bit about emperical findings.
The alloy GT40 head, the first one, with the Y in the serial number was not that great. I think some well ported E7 stockers would flow as good as these. Make sure you do not use the 65cc chamber vesion and suffer from lower compression. Or mill it.
The X version is pretty good, it flows better than the first version. Good mid-range and top end. I liked these on a 3500 lb car and a blower.
The Victor Jr. out flows either of these heads but needs a proper cam and induction to take advantage of it. Otherwise, bolting them on a mild engine will lead to losses in the low and mid range (been there, done that).
I'd select the Victor Jr. with a strong small block and good induction. The short gearing and a good cam will make the work well.
Coast has a good crate motor (302,331,342) with CNC ported Victor Jrs that flow a lot. get a good cam, good valve springs, all forged internals with H beams, girdle, and get to it.
Ron
[ June 13, 2002: Message edited by: R. Earp ]