If the engines staying in, then you need to remove the gearbox as the crank wont drop enough to do it that way.
Given whipping a CVH out takes 30-45mins max, I'd be taking it out as plus its easier to do the sump with it in front of you upside down that on your back under the car.

Risk of crap getting whipped up into the crankcase whilst your on the floor/with the underneath exposed is far greater than being able to move the block to a proper "clean" area too. All IMO/experience, again case of quick bodge fix or doing it properly. Crank bearings dont take well to an unclean environment or being manhandled due to having to pull cranks out at dodgy angles or whatever.