The zoom preamps should be fine for the mics. Loud sources like speakers don't need a ton of gain on typical dynamic mics like the 57 & 58.
The bass amp may have a direct line out that you can use, or you can use a "direct inject" box, which folks call a D.I. box, and pass the signal through it right before the amp, so you can take a line out. Acoustic guitar is most often run with D.I. rather than a mic on the speaker.
Generally when sound techs mic up a speaker cabinet, the mic is placed pretty close to the speaker cone. Speakers sound different from the edge (dull) to the center (bright and treble-y) so most time, I would split the difference and aim at a midpoint between center and edge. Listen in headphones as you move the mic while the player is playing, and you'll nail it if you have time and access to soundcheck like that.
As for mounting, if you don't have short stands, or if you had a side-address mic, you can drape the mic over the cabinet and secure it in place by wrapping the cable through the carrying handle a few times. Note that it's not ideal to mic at 90 degrees like that but it's still well within the cardioid pickup pattern of your 57 & 58, so it wouldn't be awful. A more elegant solution if you have a "stereo bar" handy is to use one side of that for the mic and clip, and just tuck the other end of the bar into the carrying handle. There are specialized L-shaped mic mounts made for this, many guitarists like the way they prevent technicians from sticking gaff tape on their vintage amps.
Buddy Guy rolls with the Audix Cab Grabber.
(edited for a smaller image, the first was enormous!)