The engine blocks are identical, so the sump is a straight swap. Thats where the similarities end, the 16v holds 4.5 litres of oil, the 8v holds 4litres of oil. Also i suspect the alloy sump was used to keep overall engine weight down and for better cooling. The ribs act like a heat sink and also give strength to the softer metal. I suspect the 16v has half a litre more oil because theres more engine to lubricate (bigger head), and the 16v uses the alloy sump and 7 row oil cooler because higher engine rpm and more heat generated.
I wouldnt do it, lets put it that way.
As for the cracked sump i found a local metal workers on a small industrial estate, they alloy welded my sump as there was about 4 hair line cracks. Cost me 20 quid.