Just thought I'd post an update... I ended up buying a radiator from a 1850 series tractor. Here is what I found: 

The Oliver 1850 radiator will fit in a 1800 once you swap over the correct radiator supports for the 1800. 

Other than the supports that are bolted to each side, the two radiators are physically the same dimension and number of cores.  

I can post up the partnumbers of the radiators for comparison if anyone is interested.

Now that I have the radiator out I might as well fix that leaky front crank seal...