You can use modeling clay if you have the heads off. When you say without ruining a head gasket do you mean by taking the heads off or torquing them down on it? If the heads are off, you can check with no head gasket, just remember that the valves are .040" closer without one. So if you are ok without a gasket, you will be fine with one.
I use a light tesion spring in place of the valve spring to check clearance. Replace the valve spring with a light tension spring from the hardware store. Adjust the rockers to proper preload. While turning the engine over you will now be able to push the valve down until it touches the piston. Find the point of least clearance (generally as the exh is closing and int is opening) and measure the distance that the valve moves before hitting the piston. You should have minimum .080" intake .100" exhaust. Let me know if you have any other questions.
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67 Fastback - Arctic White Pearl paint
351W ,Trick Flow Aluminum Heads, Edelbrock TorkerII, Carter 750 CFM, Comp. Cam 477/510 219/233@.050, Performance Automatic C-4 Trans, 3.55 gears, Front Disc Brakes, 1-1/8" Fr. 3/4" rear sway bars.
My 351W Fastback