Racking on top of the cubes is the best way not to introduce oxygen, so not sure how you plan to put them in. This is by no means the only way , just the way I do it. I pour off the spirit to a jar I can seal, then add the wood then rack the beer on top. When some time has passed(3-4 months) I sample , if it needs more of the spirit flavor I add the saved liquid at bottling. The first time I did what the directions said and dumped the whole amount in and was way too much whisky heat for a beer.