Fitting new oil pump

I am in the process of rebuilding a Breva 1100 engine.
I have just refitted the oil pump but when I tighten the screws the pump then won’t rotate. If I back off the screws it rotates again. It’s obvious what is going on here but what is the solution?
The only thing I can think of is to take the pump off again (not so easy as it’s a tight fit on the dowels) and take some material off the flat face of the gears.
And I thought this would be a straightforward job!

Well, i have spent the afternoon polishing back the face of the gears on a piece of fine wet and dry on a flat plate, until some fine stamped lines on the faces had disappeared. I test bolted the pump, without the dowels and it spins ok. So now it is bolted up properly with the dowels in place.

Do you have a gasket that goes between the oil pump and the crankcase?