mk3's don't have much room from floor to bonnet, look at the inside of the bonnet on a mk3, they have holes around where he suspension goes as its pretty close to the bonnet. Whilst on a mk4 you could fit a kitchen in between the bonnet and the suspension mount.
I swear g-werks made a sump with an oil sensor as its the only shallow sump left in my bookmarks, they stocked a few different variation, but it looks like g-werks is now out of business
Didn't VW make a shallow sump for brazilian or south african market? Alternatively your mate with the welding that did you pipes could make a oil sensor connection somehow, incase you are ever worried of smacking your sump again and it all leaking out without knowing