I've been waiting on this part:
It's the Hobao Hyper 9e Steel Uni Center Drive Shaft (89339) and its overall length is about 8.5". It should work perfectly for my front center drive shaft, giving me a wheelbase of around 18". I'll get the front bulkheads together and then be able to fit this in and see where I want to cut the front of the chassis.
Edit: Here's how it looks on the chassis with the battery trays: