Although I have not done this yet, when I was researching it for the cossie sump, I decided trimming the crossmember was easier/simplest option. It wont leak if you get it wrong either!!!

If you're worried about strength, just plate it along the bottom. Will be plenty strong enough then. I would only consider cutting the sump if you're going to mod it further, ie adding wings etc, otherwise you are creating more work for yourself and possibly creating another potential 'leaky' bit in the form of an ill fitting/leaky/deformed sump.
Just my thoughts, Shaun.