Thanks for the great input! The flex is definitely something I was thinking about. At the very least, I will isolate the cab, mid pump panel section and rear tank section into 3 separate units just like on the full size truck. We recently parked the tanker on uneven pavement and I was amazed at how much flex there was looking down the side of the truck.
I saw your carry handle in your thread - great idea! I will definitely include one.
For the drive shaft, I cut the stock Tamiya dog bone in half. It's soft enough to easily cut with a hack saw and drill. The new shaft is 3/8" dia. solid aluminum with a 15/64" hole drilled on each end. One half of the dog bone fit perfectly; no slop, but loose enough to adjust in/out easily. The other half was too tight, so I chucked the 15/64 bit backwards in my drill, put some rubbing compound on the smooth end of the drill and ran it back and forth in the hole until I got the fit I was looking for.
With the dog bones adjusted to the right length, I cross-drilled with a 5/64" bit and drove in a split pin. Using a pre-drilled 3/8 bushing in the vise helped keep the bit centered.
I thought about putting a flat spot on the dog bone and using a set screw, but this just seemed easier. We'll see how it holds up.