Spin natural.
For example you spin left and usually slide right foot forward.
When throwing a 270 on you approach the Rail from the right side as usual (or left side if you slide left foot forward) and pop of. First :Get your Tips over the rail (very important) and spin 270° .
Then slide the rest of the rail with your "unnatural" foot facing down the rail.
This unnatural stance wont affect you much if you choose a relatively short rail.(What i suggest you when you want to learn 270 on)
You are already traveling down the rail while you are in the air so you wont have to slide the whole rail in unnatural position.
I hope you get my point. I am European and im pretty tired so please disregard grammar or spelling mistakes. Thank you!
A positive side-effects of that method is that you get used to sliding down a rail with your unnatural foot facing down the rail