Your thermostat isn't the issue? I just solved my 1600 from running hot (3/4 on the gauge) by replacing the thermostat with a colder one. There was a 91 degree thermostat in it, changed to a 77 and it's running perfect now, gauge sits a bit under half round town, then up to half on they highway. Maybe worth a look?

It runs fine till a hot day comes along. On a cooler day the gauge doesn't move.

Update; I picked up 2 unknown radiators and dug out the one from my stalled Mk2 project. They are all the same and about 2 inches taller then the 1300 one, put a 2 inch block on the radiator cap and the bonnet shut. So far so good, I will now take the better one to get it checked and cleaned. What I need now is a shroud, how hard are they to come by?