Dingo Sliders - 6" from Firewall?

[cebra]

The guys putting in my motor ensured the PS and DS mount piece that mounts to the block is correct but say all the way back the engine is 6" from the firewall. I did notice my perches are in the front holes on the frame which I think is correct. Is it feasible that the pieces that mount to the perches (the other piece of the slider) is flip-flopped? Any help would be appreciated as they are at a standstill and they didn't get much support when they called Dirty Dingo (they did confirm the part number on the box was for a 67-72 C10.