340 or 360???

360 would be the way to go. They are cheap, easy to find, and perform very well.

Dougs Headers also makes a early A-body chassis headers.
http://www.summitracing.com/parts/DOU-D450/

So does TTI, both very spendy, but money well spent!
http://www.ttiexhaust.com/Header-TTI636A/63-66Abodyheaders.htm