Comfort is not necessarily an issue, but would help.
The official rule is one human, two minutes of effort. I have to get the machine from a dead stop to max RPM's in that time frame - watch the youtube video link a few posts up. We're not trying to sustain power, we want the tanks to be empty at 1:59. So in effect, it's a sprint.
The designer of the machine found a used MTB bike and it became the bike. He had a few friends/family spinning it before he found actual cyclists to spin it for him.
The frame we have is too small for both volunteer cyclists. So now we (the cyclists) want to re-engineer the power plant. We're don't want to bodge together a completely new "riding aparatus" (ie. chunk of metal w pedals), but rather drop a frame in place that suits our needs and maximizes our efforts. So we're at a point where we can start (almost) from scratch and build-to-suit. So where to start? Stiffness is a very good consideration.