If you only have room for one and want to run a stereo go with the optima or odysee(or the one with the highest amp hr rating)Yes 680 cranking amps will be fine for a small block.
I run a single sub 4 6x9s and a 600 watt amp can get about 45 min max turned up a bit but this is with just a sears die hard deep cycle grp 24.I am going to 6 vlts this year just for stereo but i have room for them and you need 2 to make 12 volts(13.5 if fully charged jeff )