The ones I have the problem is the ones in the body. Like front fender to grill area. And the part that holds the hood latch on it.Tap your holes out, chase the threads, or use a tap & die set to figure out what the thread is. The holes really should be 1/4-20. Handy is a set of socket type tap drivers, a lifesaver in many cases.
Not too many mystery threads on American cars besides some odd large threads like seat belt bolts.
Hex head "bolts" starts at 1/4-20, 5/16-18, 3/8-16, rarely 7/16-14 but I've seen it on exhaust flanges, moving on to 1/2-13. Some use of fine threads, such as 1/2-20 on oil drain plug.
Some bigger bolts underneath, K member bolts must be at least 1" thread size.
A torque chart.
Torque specifications are for stock factory type bolts with light engine oil applied to the threads and the underside of the bolt head. Other lubrications offer reduced friction and increased bolt tension, and will affect the torque figure. If using aftermarket performance bolts, like ARP, you should follow the recommended torque specifications of the aftermarket bolts.
Fastener Torque Thread Size Camshaft Lockbolt 35 7/16-14 Carburetor to Manifold Nut 7 5/16-24 Clutch Housing Bolt 30 3/8-16 Connecting Rod Nut – Plain 45 3/8-24 Crankshaft Bolt (Vibration Damper) 135 3/4-16 Cylinder Head Bolt 273-318 85 1/2-13 Cylinder Head Bolt 340-360 95 1/2-13 Flywheel Cover Bolt 100 in/lbs 1/4-20 Flywheel Housing to Block Bolt 50 Flywheel to Crankshaft Bolt 55 7/16-20 Fuel Pump to Block 30 3/8-16 Intake Manifold Bolt 35 3/8-16 Main Bearing Cap Bolt 85 1/2-13 Oil Pan Drain Plug 20 1/2-20 Oil Pan Bolt 15 5/16-20 Oil Pump Cover Bolt 95 in/lbs 1/4-20 Oil Pump to Block Bolt 35 3/8-16 Rocker Shaft Bolt 30 3/8-16 Spark Plug 30 14mm Timing Cover Bolt (cast) 30 3/8-16 Torque Converter to Flexplate Bolt 270 in/lbs 5/16-24 Vibration Damper Pulley Bolt 200 in/lbs 5/16-20 Water Pump to Block Bolt 30 3/8-16