I did quite a bit of research when it came to headers for my '64; I, too, have a mildly built 355. I found a study that showed long tube headers compared to the Dorman 2.5" outlet Rams Horn style manifolds and the results were pretty close. If you're looking for good performance and something that isn't going to turn your cab into an oven, I would consider the Rams Horn. You can get a set at Speedway for about $115. Here's a link:
http://www.speedwaymotors.com/Dorman...air,58966.html
They also make the manifold with a 45 degree kickback if you don't have a straight shot. I'm not sure what the clearance is like on your truck with that crossmember, but all I had to do on mine (with stock crossmember) was move the engine mounts to the rear mounting position.
If you do decide to go with the Rams Horn style, let me know and I'll get you a part number for the downpipe and donuts. I just picked up the downpipes and donuts this past weekend.