3ft wide gives you a lot of options really - depends ultimately on whether you want to keep with tank mates or not.
you could easily keep a redtail in that tank for a few years - just be careful on what you feed it - you don't have to feed them twice per day just to watch them grow !!!!!! - take it easy and you'll get a nice steady growth rate - the only problem you may find is with tank mates being knocked off.
L. longibarbus or pictus would be a good choice although longibarbus does get 3ft long but i reckon you wouldn't have too many problems.
To be honest it's a pretty big tank that you've got - I wouldn't get a TSN though - they're a bit too rigid when bigger.
you could get yourself a wykii, apurensis, fowleri, L alexandri, megalodoras, pterodoras, what about a few callophysus?, jurense etc etc - you've got loads of options !!!!
carl