Buy the fan clutch first, if thats not it then you are only out $100, if that IS it then problem solved.
If you drop the $400 for a new radiator and then the fan clutch is the culprit then you will need to drop another $100.
www.motorcarsltd.com has the best price I have seen on a new LR fan clutch.
Otherwise follow the link that AntiChrist posted and buy a fan clutch from Advance Auto, $65.
You will need to make the bolt holes bigger on the fan clutch but it will bolt right up no problem.
Always start free/cheap before you spend any real money.
As for testing your radiator...other than taking it to a shop and having it flow tested I have no idea.
IMO I would just replace the fan clutch first.
Now here is a tip for you, the fan clutch MUST sit vertical for a minimum of 24 hours before you install it, otherwise it will self destruct and then be no good.

 

Ooh! Good tip, I would've installed it right out of the box otherwise.

Will a radiator from a '96 bolt up okay? I might score one off a totaled disco for a much better deal.

 

Yes it will, as long as it has the same transmission as yours any DI radiator will work.
So if it is out of a DI with a auto trans it will fit, if it is out of a DI with a manual trans then no it wont fit.
But since they stopped selling manual Disco's in the US in '95...

 

Awesome. From what I understand the radiator is a combined unit with the oil cooler also built in. Just how much of a pain is it to replace the radiator? I don't want to spray oil all over my driveway!

 

Oil wont spray everywhere but it will leak everywhere.
I have yet to replace a radiator on a Disco.

 

I can't think of any reason to buy a Land Rover fan clutch instead of the one from Advance, unless you don't have a drill. http://shop.advanceautoparts.com/web...GRPCOOLAMS____
I didn't really have much mess taking my radiator.

BTW, '97 was the last year of the 5-speed in the US.

 

What exactly has to be done to make the advance fan clutch work correctly?

 

Smart @ss.



Buy the Advance fan clutch, drill the holes just big enough to fit your bolts through, done.

 

Imperial also makes a regular duty fan clutch (the one I linked to is heavy duty). It seems to engage for longer than is really needed, so the regular duty one might well do you.

 

Do I have to tap the holes or just drill 'em out?