The dip stick top level is the maximum safe fill for your sump, this level stays whatever you do with the sump because the top level is relative to the engine block/crank position.
Cutting the bottom off the sump does not change the fill level it just reduces the volume.
Adding an oil cooler will increase the amount of oil in the system but will not make anymore oil available to the pickup pipe.
Calculating the "Lost volume" is easy measure the square of the sump in mm you have removed, multiply by the depth in mm you have reduced the height/depth of the sump by, i.e 150mm x 150mm x 30mm divide by 1000 = 675 mm or 2/3 of a litre.
The usual diference between Min and Max on a dipstick is about 1 litre so you will now be on near minimum when indicating full, with the extra volume of an oil cooler no problem.
However less oil in the sump means more chance of uncovering the pickup when cornering hard.
Simple solution is a horizontal sump oil level baffle which sits just above the top oil level like a lid limiting the oil from climing up the sides of the sump, a bit more baffling can be welded verticle baffles and the best type of baffle before going dry sump is the horizontal baffle with verticle box that has 4 doors that only open inwards, these are firmly closed by the oil when it tries to excape under centrifugal force but allow oil to flow in easily
