Well, I don't have any dimensions of what you might really require but there is this:
GM UMBRELLA SEALS, 3/8"
Application: SB Chevy Truck Engines w/ 3/8" Stem exhaust valves
Specification Data:
Fits: 3/8" Valve Stems.
Inside Diameter, smallest point: .605”
Inside Diameter, widest point: .635"
Outside Diameter: .715"
Height: .450” Tall
Seal Material: Poly Acrylic
Material Temp Rating: 375F
AVAILABILITY:
But I think, you might be barking up the wrong tree with this plan!
Scott.