What kind of colour correction filters do i need?
How much exposure compensation do i need?
I know the data sheets do not suggest anything longer than 30 seconds, i want longer, like 4 minutes.
Never had a problem with exposures up to four minutes... you might get a slight colour shift (above and beyond the normal velvia shift) but it's best to test.
I wouldn't bother with anything unless you have seen problems with your particular work.
It's suggested that a magenta cc filter is used but with velvia already slightly magenta, this seems like a recipe for disaster