For a small block in warm weather I would think a group 24 dual purpose or deep cycle would be fine for starting. In a std lead acid most prefer the group 29 Interstate deep cycle (same basic size as a group 31) as their dedicated stereo battery. If you have the space for a couple of group 27 cases or trays then you can run a group 29 on both banks and alternate their function every other weekend. This scheme will give you a little longer lifespan. A single wet group 29 should give you around 100 amp/hours or roughly the playtime of two of the Optima group 34s.
If you would like an AGM you might consider the Deka SeaMate series rather than the Optimas as they provide around one third more capacity in a similar size of battery.
That's adequate for most people. Your total amplifier wattage, amplifier efficiency, how hard and long you expect to play your system at rest, will collectively determine how much battery capacity you need. A fully charged 50 amp/hours might equate to a 1000 watt total system run hard for one hour, as a crude example.
Then you want a dual-bank three-stage charger that has a total amperage capacity that is at least 10 percent of your total (both) batteries' collective amp/hour rating.