3/8" BSP is the thread size on the exhaust side of the block, under the water drain bolt (which is the same thread size too).
As said keep the hose -10 on that side, so you need a 3/8" bsp to -10 jic adapter.
i think 3/8"bsp is a bit small mate, it measures 16.9mm or 0.66"
one of ebay says its 5/8"
http://cgi.ebay.co.uk/cosworth-breat...item336050f50c