Sour Cream in Chocolate cake

I have been using my Victoria sponge recipe for my choc cakes by substituting equal amounts of flour for cocoa powder but I was wondering if (to help keep it a little moister) I add some sour cream do I need to
1 – alter any other ingredients for this additional liquid?
2 – do I now need to use bicarbonate of soda aswel? (I use self raising flour here in the UK which has baking powder and salt already added)
3 – How much do I add to a 6egg mix i.e 340g Flour/butter/sugar
4 – does the sour cream replace some of the egg or is it additional?
5 – does this help it to keep a little longer?

Sorry for all the questions but you guys are so helpful on here im sure someone will be able to answer some of these.

Has anyone every tried this or even have a similar recipe they use?