They are definately stronger, but (as stated earlier) less rotating mass. One thing to make sure of is the balancing weight. I've heard of some shafts having the balance weight stuck on with adhesive (like some wheel weights). Inland Empire Driveline has em for Mustangs and they do come balanced. Check their site here:
http://www.iedls.com/mustang.html
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347, 279/284 .493/.510, TFS TW, Stealth, 750, Flowtech hdrs, Dr. Gas X, 2 Chmbr Flowmaster, MSD 6AL-Dist.& SS coil, Art Carr C-4, Currie 9" w/3.55, alum Drv Shft, susp mods, 4 wheel power disc
In a 66 coupe