I think he is saying he has a 78" bed, not that the shaft is 78" long.
Have you set the engine and trans in place? Do you know where the back of the engine will be? That's the first step...
Once you know where the back of the engine will be start measuring from there back to the rear U-joint, Then start measuring transmissions and subtract them from the first measurement taken to come up with a rough shaft length. Now that you have a rough measurement you can ask the experts if it will work or not.
Hope this helps,
Mike B
