I just used bolts on mine.

I'm not sure the manifolds you have are the ones I was talking about and even if they are you might still want to order these and return the ones you have if you still can, if nothing else just for the cost (there 1/2 the price you paid. These require a 2 1/2" flange.

Corvette-Style 2-1/2 Inch Ram Horn Exhaust Manifolds, Pair - Free Shipping @ Speedway Motors

.