More info on your batteries is needed really.
But 5x8V is 40V, that is not divisable by 12 evenly. Best you can do is about 3 12V units for 36 volts total and 4 volt left over to waste. Yes you want 12V so that would be 3 12volt in parralel for 3X the amps at 12V, but still 4V unused.
The only 8V I am actaully familar with would be like the tractor type or those sold for old 6V collector cars. Not very pratical to mess with. More info on yours would help get better info.
Since each cell is about 2 volt in standard lead acid type batteries, deep cycle or not, in theory you could cut all buse bars and wire up anything you wanted in multiples of 2V per single cell.
Been along time since I messed with 8V for anything. 3 cells is 6V and 6 cells 12V so I geuss 8V battery is 4 cells. So you basically have 5x4=20 2V cells. If you can make the connection then you could do anything you want, 1 40V battery at equal amps of 1 cell, or 1 battey of 2V at 20X the amps of one cell.
What is worth while or pratical is something different. If the bars are on the outside of the case where you can get to them easy it may be okay to do various things. If you have to open up the battery to get to the bars I dought it's worth messing with.
If the batteries are just average starting batteries for tractors or cars it's not worth messing with in my opion either and the bars are probably not exposed or easy to get to.
