Have managed to have a first bash at packaging. A very rough start with no accurate dimensions. However, the cell sizes for the Headway LiFePO4 cells are realistic, as is the XJ600 frame. The hum Motor and tyres are random objects that need to be speed-up properly from conversations with manufacturers.
After trying a couple of configurations it would seem that 134 cells is the limit for the Headway LiFePO4. The original target was 144 cells so range would be compromised.
Michael from Microcabs suggested that I look at pouch cells a while back to help packaging. This might be a the best option to get good range.