Fit a standard series 3 radiator support panel (they allow about 25% more air flow than the earlier 2/2A ones) and a standard series radiator, ditch the engine fan, fit some electric fans either side, pushing and pulling air.
I have a 14 inch pushing at the front and two 10 inch pulling at the rear.
Up grade your alternator to a 55amp or even more. An alternative is an aluminium radiator as they disperse heat 25% more efficiently than their copper/brass cousins can, and they can be purchased reasonably cheap these days. You could get one made to order with outlets to suit the Holden donk. You can even get a series copy in aluminium here but you would need to get the outlets modified.
Land Rover Parts - RADIATOR - L/ROVER 4 cyl. 1968-84 (Copper or Aluminium)
Make sure the system is clean and run a good coolant.
If you want to increase air flow when you are travelling with a tail wind to assist with cooling, make up a couple of deflectors that will lift air from below the bumper bar and direct it up through the radiator. I have some on my vehicle and if I didn't point them out no one would know they are there. They work very well too, stops the lecci fans cutting in when travelling with a tail wind on hot day.
Fit 3.54 diff centres to slow the drivetrain speed down and reduce your engine rpm, the torquey 202 will handle it with ease. It will still even handle the overdrive, but if you find it is lacking a bit of grunt drop a Crow towing cam in, or a Holden straight six EFI cam (they are very similar) and a heavier flywheel, but they are a bit harder to set up.
As for converting to 109 six cylinder specifications to move the motor and gearbox back it would take a bit of work, relocating chassis mounting points, new bulkhead, and associated floor panels and transmission covers etc. Still not impossible though.
Cheers, Mick.

