To avoid the "infamous" flat washer you could try bolting in other shapes of metal. It may be that you just need to break up the pulsations somewhat rather than restrict them. Try a 12" or so length of perforated metal tubing, flattened gradually with the inside end pinched closed. Or the perf metal could be cut and rolled into a cone shape, tack welded and pinched shut at the end.
The total square area of holes in the perf metal should at least equal the area of the exhaust pipe outlet, causing almost no restriction but breaking up the sharp blast from each pulse. This would be almost unnoticeable, and would not involve welding on your chrome pipes.