Unfortunately, the Fords suffer from the same syndrome as the Chevys, consecutive firings on the same bank twice in the order. In order to get a true 180° header system, you will have to cross the center two tubes from side to side, Chevy or Ford.
There are two types of scavenging, primary and secondary. Primary scavenging occurs at a frequency controlled by the length of the primary tube, so having all the primary tubes the same length would bring the same scavenging intensity to all cylinders at the same time. For a good lesson in primary scavenging, look to the Jr. Dragster folks with the single cylinder motors.
Secondary scavenging occurs as a result of the coupling of scavenging signals in the collector, where the exhaust pulse from one cylinder creates scavenging in another. If the cross-sectional area of the collector is large compared to the cross-sectional area of the primary tube, you will see decreased secondary scavenging. If the timing of the pulses is irregular, you will also see decreased secondary scavenging.
The major drawbacks of most OTS headers are that the primaries are too short and the collectors are too large. A 2 or 3" inequality in primary length won't make a huge difference, but most of the rear tubes I see are about 12" shorter than the front ones. A 30" long primary tube works well with a 7000 rpm motor, but if you are trying to get more torque at 4500 rpm you will need something closer to 40".
A 1-5/8" header with a 3" collector is lacking in secondary scavenging, a 2-1/2" collector is more suited for street use. Ed Henniman of Headers by Ed has some great information on header design and selecting the best collector size. If you go to Rodney Davis' site at headerdesign.com and plug your motor stats into the calculator, you'll get a good street header design.
We use LC Engineering 1-5/8" x 2-1/4" merge collectors on our 331" high-altitude road racing motors with great success. Midrange torue is just awesome, and we recently dynoed one of them at 380 rwhp @ 6000 rpm with more than 350#ft rwt from 4500 to 5500 rpm. This was with a set of Stahl 1-5/8" x 34" primaries, a Dr. Gas 2-1/2" XCO and a full exhaust system including Dynomax Super Turbos.