Thanks to all for your feedback. I need to do a full recon. of my IV 3.5 rad. but I have an option on a V rad. on which I was contemplating doing the overhaul, and keeping the original as a spare. It looks like I need to do a side-by-side check first as it is very difficult to confirm accurate compatibility whilst installed.
I think of the tooling cost for the fronts of the different IVs. Each one has different shells, bonnets and guards. It is appropriate for the 1.5 to be unique, but hard to justify just for the subtle difference between the two radiator blocks of the 2.5 and 3.5.
I have another query. I have a larger fan available, 16", which will fit behind the 3.5 radiator with a bit to spare, and will obviously pull a lot more air. I do not know what car it was for, possibly a later early XK engine. Has anyone any experience with these? I have also seen a picture of a IV with an 8 blade fan. Maybe these were after-market options.