I just made kick panels out of 1/4" plywood and cut holes for 6" speakers in those. One thought I had was somewhere under or behind seats, depending on your seat/tank situation. My tank's out back under the bed so that was an option, if you're still stock that won't be an option. Last option was to mount facing downwards under the dash, out of sight.

'69 1/2T stepside, Sea Foam Green, 250 L6 w/ 3 on the tree, not an option on it, in the family since new.