Just bought
this battery from Ebay. It's a 4V sealed lead acid battery with 9AH of capacity. $21.50 shipped,versus $25 shipped for the 10AH one.
If my logic is correct, my Optima Yellow Top D34/78 has 55AH of capacity. It has been fine to get me to and from work. Since this new battery I'm adding in series is 4V out of the total 16, it will only be providing 1/4 of the amps to the circuit, so its 9AH * 4 = 36 effective amp hours relative to what the optima was providing. Add that to the fact that I'm only powering injectors, coil, and ecu, and it should be just fine.
Next step: determine how I want my solar charging to work for this setup!