Sorry to disappoint, but BusinessFirst is really Business, it's just to confuse everyone and make the point that it's lie flat. This means that you only get standard UC access. United GlobalFirst entitles you to arrivals access in ORD and the IFL. I'm presuming that you're continuing on somewhere? That's also a condition for access.

Alas, the United Club's have no showers at ORD other than the IFL (and in theory arrivals access for full-fare business). Alas, there are no other lounges in United's terminal at SFO. Even in the international terminal it's bad. I recently flew Etihad in full fare business from Chicago and they use the SAS lounge, which looks like it's contracted out to Ikea for furnishing and food. Alas, no showers there either.

Nonetheless, I'm sure you'll enjoy the trip. Both BF and GF are quite nice onboard.