Calvin, I'll dig out the Rohret headers tonight if possible, and see which one is the best choice.
Samdog is correct, for that sized engine and the specifications it has (especially carbs and camshaft), you can easily go with a 2.5" exhaust. Expect both a power and torque increase.
Warning! Rallycrossing is very rough on the car. You WILL remove a lot of paint from the rear quarter panels unless you have substantial mudflaps. You've been warned!
I wonder if you can actually run tall rally tires with your lowered suspension, seems to me you will tear up the front fenders at full lock. Been there, done that....
We run 13" rally tires on Gregg's (Samdog's) Ascona beater car, and they're Kumho 195/70-13's (23.74" tall), and they would rub if we hadn't cut the front valance and weren't running our taller ride height with 380 lb springs. That car has no rear swaybar at all, and the stock front swaybar has been milled down 1/4" in the center to soften it up. Realistically, 13" rally tires are hard to come by, so I'd go with 14" if you can find them. A lot of VW and Sentra rally drivers use this size, and the bolt pattern fits the Opel. Most of the 14" tires are 24" tall....15" rally tires are usually 24.5" to 25" tall.
HTH,
Bob