Bookie; most handloaders use two different powders for the .357, but not because of concerns of bbl. length. One slow powder is used for full power loads, and another, faster, powder is used for mid to low power loads.
Contrary to popular belief, the powder which reaches the highest velocity in a long bbl. will also reach the highest velocity in a short bbl. The American Rifleman addressed this issue some years ago.
I cannot emphasize too much that you should never utilize loads which are suggested by other posters, or are printed in any reloading manual that is not the latest. This is because the powder companies are retesting their data, using the latest piezoelectric method, and Winchester (Olin) significantly reduced several of their pistol loads after piezoelectric testing.
Here is how to get the latest data. Go to Shooter's Gunlinks: http://www.shooters.com/gunlinks/index.htm
Click on Product Manufacturers, non firearm. There you will find an alphabetical list where among many other things, all the powder manufacturers are listed. Alliant and Winchester (Olin) have downloadable manuals in .pdf (Adobe) format, and you can download the Adobe software right at the Win site if you don't already have it. Hodgdon has an excellent manual as well.
Once you have selected a load, be sure and pay attention to all the components; primer type and mfgr., case mfgr., and OAL.
Good luck, and reload safely. Walt Welch, NRA Life member since 1972, reloader since 1957.