Bottlehead offered a prebuilt Crack years ago where they used 100uF Audyn Q4 caps (1st photo). As you can see, they are really big... so they will take some creative mounting, as Bottlehead did on the wood base.
I personally use the Audyn Q4 68uF caps and they fit just fine (2nd photo).
Here's a chart from years ago (3rd photo) that shows the effects of lowering the output caps' capacitance depending on what impedance headphones you use. For myself, with the 68uF caps, the bass roll-off on my 600 Ohm AKG K240 Sextett will start at 4 Hz (which they can't even play down to)--and 200 Ohm headphones will roll off bass at around 12 Hz. Considering Bottlehead doesn't recommend using the Crack with lower than 200 or 300 Ohm headphones because of the damping factor, 68uF output caps make a lot of sense from what I understand.