Here is another link for performance parts for the 250 inline 6.
http://www.cliffordperformance.net/
Looks like they only have a split header for the 250. You could fab your own 6 into 1 header. on the 250 there IS 6 ports, but ports (2 & 3) and (4 & 5) only have bolt holes for the flange on the outter edges of the ports and not between them. If you look at the head and the gasket, those pairs of ports are divided by about a 1/4" of material making them separate ports.
The cast split headers on the stovebolt web site look like 4 ports on the outside, but I bet they are split on the inside...
I use to have a 6 in mine, made 250 hp easy. You could build 300 hp and still be steatable with a standard trans...