Brock also drove a Holden Monaro 427C for Garry Rogers Motorsport (GRM), winning four from four races in what was thought to be a one off appearance in the support races for the 2003 Australian Grand Prix in Melbourne. This led to him driving a 2nd Monaro built and run by GRM in the Australian Nations Cup Championship, a GT-type championship for highly modified and exotic road cars in 2003 before setting up his own team for himself and son James in 2004 (with Monaros supplied by GRM). He achieved a tenth Bathurst endurance win in 2003 at the Bathurst 24 Hour, when he won, with Greg Murphy, Jason Bright and Todd Kelly in his GRM Monaro, controversially powered by a 7.0L 427 cui V8 engine rather than the 5.7L Gen III as used by the production Monaro CV8 (the controversy came from no other car in Nations Cup being able to run a different size engine from the production model it was based on. Triple Nations Cup champion Jim Richards labelled the Monaro as a "Better V8 Supercar" when it first appeared at the 2002 Bathurst 24 Hour).