If it ends up that your best bet is to have an Adapter Plate you might be able to make it yourself or at least do part of the work.

You can use the Gasket that goes between the Oil Filter and the Engine Block as a Template so that you can mark where the holes go on the metal.

If you want to before you do any drilling you can make one side of the Metal Plate Flat by using a big File to draw file it (do an internet search on how it is done); or even better if you know some one who does wood work and has a 4 inch Belt Sander that would flatten it quick.

That brings up another method; simply lay some Sand Paper for use on metal on a flat surface grit facing up and rub your part over it and sand you single surface flat.

It is best to have a Drill Press to drill the mounting Bolt Holes; but, drilling and tapping the larger holes for the hose connections would need to go to a Machine Shop. Unless you have the right Drill Bits and Taps.

I think a 1/2 inch Aluminum Plate would work and it would sand, drill and tap easier.

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84 300D, 82 Volvo 244Gl Diesel