Normally I would agree that a 243 is a short action, but in this case I would try and call them to be certain. Short and long action in the traditional definition may not apply in this case.
The 788 didn't come in any traditional long action loadings, the longest would be 6mm Remington, which is barely longer than a .308 (2.825" vs 2.800"). It also came in (for some reason) .44 mag, which uses a
much shorter magazine than the
.308 family.
On edit, looking at Boyds site, they list dimensions:
Short action: Center to Center of Action Screws: 6 3/4"
Long Action: Center to Center of Action Screws: 7"