I would use the stock clamps. You may find that you have to do some grinding on them to get them to fit everything correctly. Sometimes the exhaust and/or intake clamp pads need some grinding massage to make the clamps sit parallel to the cylinder head. At least, that has been my experience with Fenton cast iron headers and a variety of aftermarket intakes I have built. Forget using washers. Too much strain on the exhaust system over time will cause them to bend and fail.
Tom
