Optimal Header design for 484 FE without shock towers.

Hello All,
I have a Shelby Aluminum 484 with Keith Craft CNC stage II edelbrock 6008 heads and edelbrock victor efi 4500 intake. I was wondering if anyone has suggestions on primary header tube diameter and length as well as the collector style, diameter and length or is step design better?
Does anyone know someone that makes custom headers if I decide not to do them myself?
This is a Hydraulic roller motor with FAST XFI 2.0 with ITC. This is going in 67 Fairlane 500 with C4 auto and gear vendors coupled to a 9' 3.70:1 35sp Detroit locker rearend.
When on the Dyno with the smaller Wilson 4150 throttle body and large dyno headers, the engine made 553HP @ 5600 and 555TQ @ 4900, and with FPA headers made 540HP @4600 and 547TQ @4900. I expected the HP to be over 600 for this build. I am hoping with the upgraded intake and throttle body (4500) as well as good flowing headers, I can get to 620HP or so. Cam was installed 110 @ 2 degrees advanced on timing set. Duration on cam @0.050 was 242/248, lift is 0.643/0.637, lobe sep. 113.
Intake/Exhaust valve size is 2.19/1.75" with port volume best I could find online is believed to be 165/130cc and 76cc chamber. Posted head flow numbers at 0.3 lift is 219/163, 0.4 278/196, 0.5 311/220, 0.6 324/240, and at 0.7 335/253. Please let me know if you have any suggestions or need any more info. This is a 4.255 bore, 4.25" stroke motor at ~10.6:1 compression. I will probably install cutouts immediately after header if you believe that will help performance.
Thanks, Josh

Josh, you might check out these guys - Stainless Headers Mfg., Inc. fabricates custom headers in stainless and mild steel