Pretty much any set of headers for the six are going to be split by design. Clifford claims 18%-22% power gains over stock with both thier shorty and long tube headers. I'm not sure of the difference in power versus the two. If you really want a strong 250, head work is where to spend your money, the stock design just doesn't flow well at all. With head work and headers alone, you'll notice a night and day difference.
Here's a pic of my motor and some video just because I like posting it.
Specs are:
250 L6 .040 over
Cam and Z28 valve springs (forgot the cam size, but it's somewhere in the middle, turbo friendly!)
Ported heads with head bolt bosses removed
Clifford shorty headers 2" exhaust
Offenhauser intake with holley 390 cfm carb
HEI ignition
4 speed