You should take the firing order of your engine. Devide it in half using every other number. The 1st/3rd/5th/7thThats Chev- Ford is 1234 on RH Bank & 5678 on LH Bank is the four for the side of the engine that has #1 cylinder on. The other four are for the other side. They should fire in sequence in the collector. This allows the firing cylender to scavange the previously fired cylinder. each primary should be as close in length as possible. the length of the primary's determine where your power band is. I made mine by hand and they are all within 2 inches of each other.
Ford V8
260, 289, 302(5.0L)
up to and including 1984
Firing Order
15426378
To get the propper paring you put the two cyls together that are 180* apart, so it is
1542
6378
1-6, 5-3, 4-7, 2-8 Then you gather them into the collectors (1653) (4728), there you have it a bundle of snakes with 6 & 7 swapping sides