Really interesting and helpful, thanks.
The Comma Xstream G30 that I was going to use in the XT seems to be fine - it's also fairly cheap in concentrate form.
http://www.commaoil.com/productsguide/view/6/351
For flushing I use a garden hose, then to flush that out I use water I've saved from my dehumidifier, then anti-freeze/distilled water 50/50 mix.
The car (1965) can't use OAT type anti-freeze as it can find any porosity in the head and block - not just creeping through gaskets, actually through casting imperfections in the block! With catastrophic results.
The advice is not to use OAT or HOAT anti-freeze in old cars (anything before about 1975) - which just leaves good old Bluecol (or the Comma version of it).
Also, I don't tend to use distilled water in the car, just a 50/50 mix of anti-freeze and dehumidifier water, as it needs the best part of two gallons every two years!