You don't have enough clearance between the gears and the plate. The easiest way to solve this is to use a thicker gasket. I have kept every unused oil pump gasket that I have ever gotten for just this purpose. Granted, I'm in the automotive machine business so I've gotten quit a few over the years. Usually you get one in the complete gasket set and you get one or two in an oil pump kit. Every gasket manufacturer seems to use a different thickness for this gasket. You can measure the thickness of the gaskets with a micrometer. Setting the clearance on these pumps is one of the most important parts to making the Buick oil system work. Usually the gears protrude from the housing by a couple thousandths of an inch. This can be measured using a small straight edge and some feeler gauges. If the gears protrude .003" use a gasket that measures .005" to .006" if you can find one. I believe that the factory calls for .002" to .006" clearance. Tighter is better but too tight is bad also. In cold weather you can get into trouble if the housing shrinks and pinches the gears. I usually set the pumps up fairly tight and then put the assembled housing into the deep freezer overnight. After several hours I pull it out and make sure the pump still turns. I got burned on this once is why I do it. Did a job in July and the guy took the car straight home, put it in the garage and didn't pull it out again 'till new years eve. Pump was locked tight as a drum at zero degrees. TA Performance sells a selection of gaskets of differing thickness for this purpose. If you are not inclined to get the right gasket, you can can dress a little off the end of the gears to obtain the clearance that you need. A sheet of medium grit wet or dry abrasive paper on a piece of glass for a flat surface will work for taking a little off the gears. I highly recommend using the thicker gasket method as dressing down the gears evenly is difficult and requires good measuring tools to make sure it's done right. Plain old plastigage, available at any parts store, works real good for measuring the assembled clearance of the pump. I would be inclined to dress the booster plate face on the wet or dry paper also to make sure that it is flat. A high spot on the plate could cause problems also. I hope this is enough info to get you pointed in the right direction. Resist the temptation to just put it together and hope that it just wears in. You will likely have aggressive distributor gear wear and could take out the front cam bearing. Which will ruin your day right quick.
