An 11" x 11" x 1.5" Tru-Cool H7B sells for about $72 from several vendors.
This same cooler is sold under several brands and is an excellent product.
It is worth that much money just to know the inside of the cooler is clean
and not going to shed chunks of mineral crud directly into my bearings.  I
would not trust even a new heater core to be clean, let alone a 20+ year old
one.  You know, of course, the oil filter is suppose to go upstream from the
oil cooler.

There are several oil cooler installation ideas in the SubaruVanagon Yahoo
list Files area.

Ed