[WE7X]

You are testing cylinder compression (basically rings, valves and head gasket sealing), so the fuel pump should not enter into the discussion.
Disconnecting the electric fuel pump will not hurt anything, but is a bit of extra effort.
The fuel pump only refills the carburetor float bowl. If all is working as it should. the only fuel being taken from the float bowl will be as a result of the air being pulled into the engine as a result of the piston and valve action ( the engine is basically a large air pump, to which you add fuel and a spark.....hopefully at the correct volume and time).
By having the throttle wide open, it reduces the vacuum in the manifold and carb, and will draw somewhat less fuel into the manifold and cylinders while cranking. That should not make any significant difference in your compression readings. In general, having the wide open throttle might make readings a bit higher, because there will be less restriction in the air passage. In practice, it is not particularly important, because most of the time you are looking more at relative readings between cylinders; and different compression gauges and procedures will give slightly different results also.

Try to run each cylinder through the same number of compression strokes to be consistent. Usually five of six compression cycles is plenty in a decent motor. Some guys do more, looking for the absolute highest reading. Personally I like to see what happens by the time a motor has gone through three or four compression cycles. Hopefully by four, you will be very close to the maximum pressure. If it is still climbing significantly by the end of four cycles, I would suspect weak rings; but there are a lot of other variables involved.
If you suspect weak rings, the common test is to add a teaspoon of motor oil and retry the test. That helps to temporarily seal the rings an will usually result in higher readings. If the readings go up a lot, then there is likely a ring problem, but just very minor increase could be attributed to the added oil taking up a bit of the combustion chamber volume.
It is all relative, so don't hang your hat on any specific numbers. If it starts OK, and runs relatively smoothly, does not burn a lot of oil, and seems reliable; you should be good to go with it for quite a while.
There will likely be other things to take up your spare time and provide more 'education' on big American iron.

Rod J
Issaquah, WA