Well, it looks like I pulled a bonehead move. First of all, did I mention it's really cramped in there and hard to see anything? Yeah, well I put the screws back into empty holes last night, but one was just a threaded hole about a half inch away from the modulator bolt hole. I guess that one has a use on another version of the transmission, but on mine it's just a sucker hole. I might have been able to just move the screw, but I took it all apart again to check the o-ring. It seemed to be seated fine. It was a LOT easier with the flex disc removed. The thing didn't want to drop very far anymore, so I had to wedge a 2x2 in the driver's side.
It is hard to push the new modulator in, but I rocked and twisted it a little, and it seemed to go in. Starting it up again, no leak. Now to check the pressure...

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1982 300CD Petrol/Black Leather