Th400 on driveshaft a g body

[Link]

I am doing a th350 to th400 in my car and I need to get my driveshaft shortend can some one tell my how long it needs to be btw it's a 79 El Camino

 

[565bbchevy]

The most accurate way is to measure it,
http://www.dennysdriveshaft.com/how_to_measure.html