Stock heads:
Bigger valves = NO - SBC = 1.74-2.02", 273/318 = 1.78” 340/360 = 1.88-2.02", Exh SBC = 1.5-1.6", 273/318 = 1.5”, 340/360 = 1.60"
Thinner stems - yes & no compared to 273-360 LA 3/8 yes, the 318/360 Magnum at 5/16" is thinner than SBC 11/32". LA valves are stepped down...