Thougt I'd throw in some visuals:
Below is a google maps visual of the planned Transbay Redevelopment Area.
The Transbay Center (1000+ ft) is the big grey box in the middle with the bus lines running to and from it. First and Mission (850+ ft) has a marker on it, and I believe the Howard & 1st parcel is the parking lot above the word 'Tehama' near First. (FYI - 301 Mission (Millennium) is at the corner of Fremont and Mission, right where the 't' in "50 Fremont Center" is, 300 Spear (Infinity) is at Spear/Folsom/Main, One Rincon Hill is at at the bottom of the picture at 1st & Harrison)
We've seen the Transbay Terminal before, so it wasn't really exciting to capture the parcel, but here are what I believe are the two 850+ ft. towers that are being discussed.
The first one is at the Northwest corner of 1st and Mission, and its footprint will extend to the parking lot. This is the view from Local Live looking East, I believe. The white lowrise building is the one being talked about, and the adjacent abandoned parking lot above it (which by the way smells offensively like human feces everytime the sun comes out).
The Howard/1st parcel, I think is this one. I cannot see where else they would be able to easily build, unless they are tearing down the building. My opinion is, if the parking lot goes, I'm for it, whatever it is! This is the view from Local Live looking south. From memory, I believe the building in the lower right corner is part of the Foundry Square complex, and is also the corporate headquarters of Gymboree.
Hope this sheds light for those who had difficulty imagining the actual sites on the city grid.