2 Radiators?

It will work, but there are probably easier methods.

Friend of mine has all his series LR's kitted out for heating in the back in the form of 2 foot long by 6 inch wide radiator cores, these are fed from the heater matrix circuit and it's a 109. These pumps brute force and ignorance jobs, there was no planning they just bolt on and pump, LOTS of water. In your case running full bore rad pipes would be an easy route, you will need to modify the expansion tank - i.e. raise it.

A lot of hybrid offroaders, including 2 V8's I know of locally, have the rad in the back, with twin electrics drawing air from a scoop at the front, over the roof and through the rad, and again no problems - probably a lot of work though.

I have done this, on a mini, it had a honda r type vtec engine, and the rad would not go in the front.
The pump on the engine did not have the balls, as most wouldnt I think, I ended up with a rally water pump which was electrically powered, this did the trick ok with the long pipes to the boot.
If you just want heat in the back, have you considered something like an eberspacher diesel night heater?, these are popular for heat on boats.