I am thinking of fitting a pair of these to my Westfield wide track:

http://www.rallydesign.co.uk/product_info.php?products_id=11815

They recommend a Transit drive link ball joint:

http://www.rallydesign.co.uk/product_info.php?cPath=1375_1594_1595&products_id=11838

My current upper wishbone appears to be 1/2" UNF ? What thread is on the Transit part?

I believe its 18mm 1.5 pitch
I stand to be corrected that's from memory

Hi mate, I have sent you over a U2U about the kits I supply to fit the RD upright. I should be a registered trader on here as well but it hasn't been activated yet.

I was about to say why not use a 1/2" rose joint conversion

The joints I have here are M18-1.5

They're of varying quality, so be careful, lots of really terrible crap out there. Some of the dire stuff can be from sources you might consider safe, or at least I did.

Lemforder are the best I've fitted, readily available. Raced ruthlessly, hassle free.