I think most of the US Westerns which were made at the same time of the Spags are not different in regard of dirtyness or pessimism. The US western was always progressing from 1939 to 1976, and so was the depiction of violence and a generally darker atmosphere.
The assumption that US Westerns were always clean and nice with the white dressed hero fighting the black dressed baddie was not even really true for the 40s and 50s and surely not anymore in the 60s, long before the SW arrived.
So you can actually watch most of the better known US Westerns of the late 60 and 70 without risking to end with a clean cut cowboy movie.
But here are some with which you can’t do that much wrong:
The Wild Bunch
Pat Garrett & Billy the Kid
Little Big Man
Butch Cassidy & the Sundance Kid
One Eyed Jacks
True Grit (1968)
Ride the High Country
The Life and Times of Judge Roy Bean
Dirty Little Billy