What are the differences between models M and J on GM 3100 V-6s?

Vin J is typically was an aluminum head motor made from 1999 and newer for some models. 1994-96 3100 SFI V6 vin M ENHANCED /160hp @ 5200 rpm /185tq @ 4000 rpm /9.6:1 Compression /FWD, distributor shaft driven oil pump, DIS, hydraulic roller camshaft. 1997-04? 3100 SFI V6 vin M RPO LG8 /175hp @ 5200 rpm /195tq @ 4000 rpm /9.6:1 Compression /FWD, distributor shaft driven oil pump, DIS, hydraulic roller camshaft, roller rocker arms