I just purchased an in-line 1 3/4" radiator coolant filter that I believe should work on the top hose between the thermostat housing and the radiator inlet on this 218 flathead six engine. There's a few brands out there, but to my knowledge this is the only one that is small enough to insert between the radiator inlet and the thermostat housing, as the entire unit measures only 3 5/8 " wide and requires that 2" of hose be removed (cut out)..filter has removable cap for servicing.....if interested - pm me and I will share the info. I'm not connected or involved with these in any way, other than I had been trying to obtain one, but they were only available out of Australia and the shipping made them crazy high...however now there is a distributor here in Texas which brought the price way down....not trying to violate any forum rules, by advertising for anyone,...just wanting to share with folks that might have a need..
Steve