Sure, you can fit a shorty LiPo pack. The battery compartment is sized for a full-size LiPo pack, so you'd just stuff the remaining space with foam to keep a shorty pack from rattling around.

Mounting the front gearbox in the rear was slightly more complicated than I expected, but not a Herculean effort. The bottom part of each gearbox is keyed to fit only in the front or the rear, and the top part of each gearbox is keyed to fit only its corresponding bottom part, but some careful cutting can make the top half of the front gearbox fit the bottom half of the rear gearbox, and vice-versa. Then you have to drill new holes for screws to go completely through the mounting tabs that hold the top and bottom parts together, so you can secure them with screws and nuts. The piece that attaches the front gearbox to the top of the main chassis also has some screw holes that don't quite line up with the top part of the rear gearbox, and the piece that attaches the rear gearbox to the top of the main chassis has some screw holes that don't quite line up with the top part of the front gearbox, but if you cut the screw holes so they become slots instead, you can still pass screws through to anchor everything together properly.

I wish Tamiya had just made the front and rear gearboxes completely interchangeable, but the modifications needed to swap the front and rear gearboxes are doable in a single afternoon.