Cool, thats prob simpliest. Bar will reach 8th cell set in between 2 cells on the bottom row (usual way) or if you have to can place 8th cell directly on top of cell at the end. I've always used something like shoe goo to help hold cells together - I don't reply on just the bars, but I don't use an external cover of heat shrink either.

Build the 7 cells s-b-s, the decide which end is best for the 8th cell, may not make a diff. Look at his chassis and see which end of the pac the wiring needs to be on. One way to wire this - one of the main wires will come off the bottom row, the other off opposite polarity side on the top cell. The other way, you'll have 8th cell connected by two bars and main power wires will both come off the bottom row at the other end of the pac.