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The early 97-98 had perimeter type bolt pattern on the valve covers, 99+ are centerbolt. The later model heads flow a little better out of the box. The early model engine blocks are slightly weaker between the cam and crankshaft centerline. Later models have more aluminum in those areas to strengthen the block. The later blocks also have larger oil ventilation to get the oil out of the valve train and back into the oil pan. 2001+ intakes are LS6 flat bottom design and have about a 15HP bonus. 2002 was the best fbody LS1. 2001-2004 LS1s are available from GTOs and Vettes and are great engines. Be sure to try to get 243 head castings if you can. 241s are good, also.
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