I want to get rid of my hedders, and put on some manifolds. I drive the car alot, I don't race. I want some manifolds that will perform well. The HP 340 manifolds are too expensive. Do I have any other choices that will fit? Any newer 360 manifolds that will fit in a Duster? Something that is cheap and easy to find in a wrecking yard?
I have a 70 Duster with a 318.  

I'm not sure if anyone has done it, but 360 truck manifolds are almost as good as 340 HP's. They are a center dump manifold. It might get a little tight on the driver's side around the collector if you have Power
Steering. A good pipe bending muffler shop might be able to help in that area. I've thought about doing the same thing you want to do because I don't want to spend the money for the TTI's either. There is a tech article on hedders and manifolds tested on a 300 horse crate motor somewhere on this site with the 340 hp and the 360 truck logs.  

Thanks for your reply. I read that article on the crate motor, and you are right those manifolds did pretty good. I do have power steering, and it looks like a rear dump would work better. I am hopeing to find someone that has installed those center dump, or tryed to install them on their duster, before I Take off my hedders.
I had a pair of those Truck logs once and threw them away. I wish I did not do that.