Do you already have a driveshaft? Are you changing the rearends?
If you already have a driveshaft and you aren't changing rearends, you should measure from the end of the output shaft on the trans to the end of the pinion yoke on the rearend and tell them you are running a TH400. Which 4speed do you have? If it is the SM465, I can't remember if it was the TH400 style output.