Hi Richard, so far Ron has been right on the button 👍 and just beat me to it on the planking. BTW: Planning is half the game, so first split the planking packs into two equal sets. Offer up the shorter planks to the framing to see where to cut with absolute minimum wastage.
MEASURE TWICE OR THRICE AND CUT ONCE!! is the maxim😉
Work on alternate sides to minimise twisting of the hull. Good luck.
The second batt Ron suggests should be your solution as it's only about 68mm long. The 'inline' pack you have is about 105mm.
https://www.componentshop.co.uk/7-2v-3700mah-sc-nimh-compact-battery-pack-hbz6512.html
Actually the Billing instructions (such as they are🤔) recommend an 8.4V pack, but since you have fitted a more powerful motor than recommended this 7.2V pack should throttle it back a bit 😉 Run time probably less than with the 400 though.
Cheers Doug 😎
PS Dimensions of Hump Pack added; too long! 112mm😆