Have you verified that all of your “charging” sources can be adjusted to the needed BB lithium settings? Some reworking of the battery interconnect cables will probably be needed going from 6volt in series/parallel to 12 volt in parallel. Will you be able to mount the BB’s in a area that you can control the low/high extreme temps?
(BB’s are a bit discounted for “Black Friday” and “Cyber Monday” on their website.)