a feeler guage cannot be calibrated, it is made at precise measurements that cannot be changed.
When a feeler guage is out of spec, it is replaced.
You do not have to calibrate feeler guages. They are made to extremely fine tolerance and nothing you could do would improve them.
With a straight edge and feeler gauges.
Feeler gauges.
Checking the spark gap on spark plugs, and other small clearances
using master vacuum gauge and master pressure gauge
Nonmetallic feeler gauges are used by technicians to measure the air gap that exists between the coil body and the armature on an electro-magnetic brake. This type of braking system generally can be found on industrial cranes and on marine davit hoist systems.
They work well for checking gaps and clearances such as the amount of backlash between the cam and lifters.
The important point in feer gauges ( also called thickness gages by some of the manufacturers ) is the thickness of the blades. Therefore in order to calibrate a feeler gauge you need to measure the thickness of the gauge at various locations along the length of the device. The standard used in the calibration should satisfy a TUR ( Test Uncertainty Ratio ) of at least 4:1 in order to limit the FAR ( False Accept Risk ) to nearly 2%. Such devices do not require periodic calibration. However, they should be standardized before every use by a standard in order to assure reliable measurements.
An electronic way of setting the gap in ignition points. This is more accurate than using feeler gauges.
Stock 350's have hydraulic lifters, so feeler gauges aren't needed. Get yourself a Chilton's manual or similar and read up on the valve adjustment process.
Valve clearances are measured with feeler gauges on cold aircooled engines or warm water cooled engines.
If you're asking about feeler gauges, then a pair of 0.002 plus an 0.001 would do, but make sure they are very clean and undamaged.
You will need feeler gauges, a ring spacer and separation tool, the correct specs for their correct placement on the piston and the gap specs.