Art Vandeley and I had the following dialog yesterday:
me: "Replacing the oil pump requires disassembling the entire engine."
Art: "What??? I changed the oil pump in my Spider a couple of months ago. I only had to remove the sump guard and bottom section of the sump."
In thinking about it, I agree with some of what Art is saying. A few years back, I had tried to remove the ENTIRE oil pan with the engine in place. I got it unbolted OK, but when I tried to slide it forward, it hung up on the oil pump. So, in my mind, the pan couldn't be removed with the engine installed. But, Art points out that the lower part of the pan - the sump - can be removed, giving access to the three bolts holding in the oil pump, which would allow the pump to be removed. OK, I stand corrected.
However, I still maintain that it is chancy to bolt in an oil pump without checking that its shaft isn't binding in the bore in the front cover. And, it's tough to do this with the engine assembled, because the gears between the pump and crankshaft prevent you from rotating just the pump to make sure it turns freely. Just bolting in a new pump is the sort of thing that might work OK 7 times out of 10. But, sometimes you need to fiddle with the relative tightness of the 3 bolts to get things aligned properly.
If you have a loose front cover & oil pump in your garage, try it. Bolt the pump on with an O ring, torquing the 3 bolts tight. Try to turn the pump shaft with a large screwdriver in the distributor drive cogs. Many times it will be very tight. If you unknowingly assemble an engine with this condition, the result will be the oil pump drive shaft shearing after a few hundred miles. They always break at the hole in the shaft where the roll pin goes that secures the drive gear to the shaft.