Are you looking to buy a used one or make a new one?
are you using small U joints or large?
either way the only correct answer is for you to measure your particular situation
You can be sure you are in the ball park with your measurements by downloading your cars service manual and looking at the propeller shaft section and /or downloading the parts book and doing the same. That is, if your car came with that combo as an option.
Maybe you get lucky and someone else has the same combo but maybe they used springs that moved the rear back an inch or two? I would hate to layout $150 on a driveshaft only to find its too short.
As you know, the 340 did not exist in 1963 and only came equipped with a 727 trans. The tail shaft lengths and types have changed thorough out the years since the early sixties into the seventies so it will help to decode the date of your trans. Dennys Driveshaft How To Measure Correctly for driveshafts that fit right