Nice to hear of another 55/409 project!
My 55 has a stroker 409/476, and has had the "W" implant since 2008. Depending upon how "serious" you want for a motor, and
how you plan to use the car... ie cruising, shows, or racing, etc. will define your style of headers. I used my 55 solely for driving
around, shows (won two 1st prize Super Chevy shows in 2006 with it), and found that the stock 65 Chevy headers work nicely
in my 55 (and they are smoothed and ceramic coated).
But, if you are not using a rack & pinion steering setup, you will need to reshape the after-part of the left header a bit,
to clear a stock 605 steering box. I use the rack & pinion. My engine is also mounted 3/4" forward of stock (and they should not be
mounted any further forward than that, PERIOD!
Also, a nice alternative, is sectioning in a custom G-body frame clip, which brings all the steering gear forward of the front suspension
and allows for a bolt-in G-Body cross-flow radiator (comes with a custom radiator support), and accepts all stock or after-market G-Body suspension AND allows for a far larger engine bay for header clearance! There have been some poo-poo's posted about this setup, except the fact is, it is a great setup, not difficult to do, and the tri-5 handling is greatly improved.
Problem is, I do not believe the clips are available any longer. ... BUT..., I do have one, never installed and FRESH!!!
I'll post pictures, if you'd like to see it. I may or not use it when I get back to refreshing my red-white 55 in the avitar.
I hope this post helps you.
Denis (DeSapience)