draining engine oil depends on how fast your tank leaks down. Personally I have done it drained and filled its easier drained. Or at least drain the sump if you think your going to be quick about it. Also I would use stock fibers before I bought barnet ones. I use the MTC fibers and will never go back. If you choose the mtc fibers leave out the anti- judder spring and seat out. The rear fiber has less surface area due to the anti judder crap. MTC uses full fibers all the way thru. I also would add stiffer springs, MTC has some stronger ones which are ok if your beast is not a big hp monger. Heavier duty springs help the engine that their owners run car oils with extra friction modifiers in. (not used in wet clutch situations.)
You should also update the lifter piece to the newer style it has a larger head on it compared to the 06 style.