Much of this bulb info, and other info applies to P and G model Chevy/GMC vans too.

LAMP AND BULB DATA (From my 1977 GM Factory Service Manual)

USED FOR		QUANTITY	TRADE #	(CANDLE) POWER
DOME LAMPS; CAB		1	1004	15 CP
Suburban and Utility		1	211-2	12 CP
Generator indicator lamp		1	168	3 CP
Instrument Cluster lamps^		5	168	3 CP
Headlamp Beam indicator lamp		1	168	3 CP
Lamp assembly - Tail &Stop lamp		2	1157	3 -32 CP
License lamp~		1	67	4 CP
Directional Signal (front marker lamps)		2	1157	3-32
Head Lamps		2	6014	50-60 W
Temperature Indicator lamp		1	168	3 CP
Clearance and Marker lamps		4	168	3 CP
Roof Marker lamps		5	194	2CP
Brake warning indicator		1	168	3 CP
Transmission Control (auto)		1	1445	0.7 CP
Backing lamp (reverse lights)		2	1156	32CP
Heater A/C		1	161	1 CP
Cargo lamp		1	1142	21 CP
Cruise Control lamp		1	53	1 CP
Courtesy lamp		1	1003	15 CP
Windshield Wiper switch		1	161	1 CP
Clock		1	168	3 CP
Rear Identification`		10	1895	2 CP
Under hood lamp		1	93	15 CP
Seat Belt warning		1	168	3 CP
Cargo/dome lamp		2	211-2	12 CP
Four Wheel Drive		1	168	3 CP
Oil Pressure Indicator lamp		1	168	3 CP

* On CA, KA 10-35 instrument clusters only
^ 3 lamps used on instrument cluster on models without gauges
~ 2 lamps used with step bumper and on P models
` On Wideside Pickups
All headlamps are sealed beam 50 watt low beam, 60 watt high beam.

CIRCUIT			FUSE
Heater, Front A/C, Generator Warning Lamp			20 Amp
Idle Stop Solenoid, Aux. Battery, Radio, Time Delay Relay, Emission Control Solenoid, Transmission Downshift (M40)			15 Amp
Cigarette Lighter, Clock, Dome Lamp, Cargo Lamp			20 Amp
Fuel gauge, Brake Warning Lamp, Temperature Warning Lamp, Oil Pressure Warning Lamp			4 Amp
Courtesy Lamp, Roof Marker Lamp, License Plate Lamp, Parking Lamp, Side Marker Lamp, Tail Lamp, Clearance Lamp			20 Amp
Directional Signal Indicator Lamp, Stop Lamp, Traffic Hazzard			15 Amp
Instrument Cluster Lamp, Heater Dial Lamp, Radio Dial Lamp, Cruise Control Lamp, Windshield Wiper Switch Lamp			4 Amp
Windshield Wiper/Washer			25 Amp
Cruise Control. Rear Window Aux., Fuel tank, Tachometer, Back-up Lamp, Directional Signal Indicator Lamp, Directional Signal Lamp, Headlamp Buzzer			15 Amp

The truck's wiring is protected from short circuits by fuses, breakers, and fusible thermal links. The above table shows what amperage fuse should be used, with each respective circuit. In a pinch, a lower amperage fuse can be used to replace a higher amperage fuse. NEVER replace a fuse with a higher amperage fuse. SEVERE wiring damage can occur.
In-line fuses are located in the auxiliary heater circuits, under hood lamp, and front and rear A/C circuits.
Circuit breakers
Headlamp and parking lamp circuit Amps: 15, Location: light switch
Tailgate window motor Amps: 30, Location: under dash
In addition to a fuse, the Headlamp circuits are protected by the Circuit Breaker in the light switch. An electrical overload on the breaker will cause the lights to go on and off, or in some cases to remain off. The windshield wiper motor is also protected by a circuit breaker. If the motor overheats, due to overloading caused by heavy snow, etc., the wipers will remain stopped until the motor cools.
The following wiring harnesses are protected by a "fusible link" which is a special wire incorporated in the circuits:

• Headlamp high beam indicator
• Horn
• Air conditioning high blower
• Ignition circuits
• Starter Solenoid (pull in and hold) circuit