I can find no specific mention of a gasket for a Chevrolet engine in either the 1958 shop manual or the Motors manual for 55-62 261s. The GMC shop manual for 47-54 does specifically call for the gasket and the cam slips through it after the gear and thrust plate are assembled.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.