Nicked from another forum, thanks Kat!
After measuring the Viva ones and finding them to be 3/8" NPT, an American pipe thread, I figured it might be easier to find something for an American engine.
A bit of research and I find: 1965 - 1969 Chrysler big block and 426 Hemi engines - Fittings consist of two 5/8" hose fittings, one is 3" and the other is 1-1/2" long. Thread is 3/8" NPT. Sounds good.
Pic shows the two types of Viva ones I've encountered with the nice shiny new Chrysler ones (you get both; although the Viva only needs one I don't see this as a problem as Viva ones seem impossible to find.) I haven't tried them yet but the threads match the old ones. Problem solved, methinks
I got them from Gotham City Muscle Cars on eBay.
Look for "64 65 66 67 68 69 426 HEMI & BB HEATER HOSE NIPPLE SET".
Ordered last Saturday, they arrived today (which is quicker than I've had some stuff from the UK.) Price ($15 with $6 P&P) seemed reasonable as I once lost a bidding war for a slightly less knackered original Viva one which went for over £20!
http://stores.ebay.co.uk/Gotham-City-Muscle-Cars