It's strictly a trademark issue. I just talked to D&D the other day about this and was told they still have quite a few Viper T56 trannies in stock for the foreseeable future. However,they are in negotiations with Chrysler to resolve the problem.
I've got one in my 69 El Camino and it works really well with 4.11 rear cogs.
6th gear is strictly for highway fuel economy.

If you have the funds I have a great idea..

How about a TKO 5 speed and a gear vendors overdrive?

I didn't even know they make them to fit on manual transmissions. the 5 speed will be a lot stronger than the 6 speed and the OD will get you the milage.

You will also have more gear selections.. This is what I am thinking of doing on the 68 since the torque of the big block would beat a T56 to death (and I already own a gear vendors unit).

Might be a good option if you are going to be making lots of hp/tq..

For that matter you could do a T10 (inexpensive) and a gear vendors.. od would be .78 i think.. dont quote me though :)

Or if you like you M20 you could hook a GV to it as well.. I think they are good for 1100hp and pretty much never brake. You would need to shorten the driveshaft though.. I know the unit is a bit price but it is an option and it also has almost zero drivetrain loss (though it does ad weight)

A Gear Venders unit might be good for 1100hp but I'm not sure T10 or M20 could handle half that much. :)

The 5 speed could as it is much stronger than any 6 speed.. What is a M20 good up to with high end parts? I know they can make some strong T10's as well..

Just thinking out loud..