Hi Steve,
I'm using Bilt hamber dynax s50 for my rebuild. I was very pleased with their water soluble de-rust solution (deox-c) and their rust converter (hydrate 90) so I decided to stick with them for the cavity protection. Bilt hamber get good write ups and are a UK company (Billericay, Essex) so it will be easy for you to get a hold of. I think that there is three different formulations depending on if it will be exposed to UV light etc.
ACF-50 (Lear corp)also gets good reviews but is slightly harder for me to get a hold of here in Ireland but that is improving as I write. I got a small tester sample at my local motor factors and they said they could get it if I needed it but it's not something they would carry in the shop all the time. I'm using the Lear corrosion block grease during my chassis rebuild. Early days yet but I shall report of the results in time.
Regards,
Pete