Hi John,
Does the oil pump run off the mag on the Buick? Pretty sure it does, but I don't remember. If it does, check to be sure it's really going down all the way and if there's a pump tang, that the radius of the cut on the tang isn't holding it up.
Next thing to do would be to pack the gear teeth with grease and insert the mag as far down as it will go. Pull it out and see where the cam is engaging with the mag. Should be about in the middle. If it's low, I'd suspect the oil pump issue...if it's in the middle or high, maybe the shoulder was machined in the wrong location.
Check the length against one of your others. Maybe it's just the shoulder. If so, we can machine that off and put a slip collar on it for you. Just have to make sure the oil gallery grooves and what not are going to work or damage to other things could result.
Spud