A  1600/2Litre has a larger coolant/cooling capacity due to the extra length from the top to the bottom of the radiator, to the bottom of the Radiator. ... As you wish to run a High Performance 1600 Engine you need a High Performance Cooling System, a Triple Core will give approx. 30% more cooling capacity, the radiator fan shroud is assisting in drawing air through more of the radiators Area.!!
 
High Performance can mean, I want a Full Race Cam, Twin 4 barrel carbs, 12-1 Compression Ratio, you would need to run Race Fuel, or at Least, AV gas, engine build cost may be $10,000, Destroyed by an inadequate Cooling System.!! ... You need to list your build details, then ask the Question Who can Build me a Radiator/Cooling System to Cope.!!
 
Does that make sense,???

For that spec a standard 1600/2000 radiator will be more than enough. Or just triple core the 1300 radiator so you don't have to cut the car up

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?

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?