larry heres a link.
http://www.markwilliams.com/driveshafttech.aspx sounds like a 3.5'' shaft at 54'' is rated to roughly 6700 rpm. I'm guessing you'll turn it more than that. the 4'' will get you another 1000 rpm. and I would suggest the bonded and not the welded aluminum. according to m/w it's a much truer shaft.