Sanderson makes mid length headers, that is what I will use on my 454.
If it has a Q-jet and you want good mileage, don't change it, rebuild it.
Is it a dually? With the gearing, and a 454 12 is going to be about the best you will get out of it keeping speeds around 55mph, you may squeeze more, but it won't be much more. If you invest in a Overdrive trans it may help some.