How do you remove rear break drums from 1976 chevy 4x4 c20

These?

https://www.4wheelparts.com/b/suspension/control-arm-drop-bracket/_/N-cm7od

Specialist suppliers here

https://www.google.com/search?q=1994+Chevy+k1500+4x4+front+lower+control+arm+rear+suspension+drop+bracket&tbm=isch&ved=2ahUKEwii472E5MD1AhWHq3IEHdO_CeoQ2-cCegQIABAA&oq=1994+Chevy+k1500+4x4+front+lower+control+arm+rear+suspension+drop+bracket&gs_lcp=CgNpbWcQAzIHCCMQ7wMQJ1AAWABggQtoAHAAeACAAVmIAVmSAQExmAEAqgELZ3dzLXdpei1pbWfAAQE&sclient=img&ei=CJLpYaK_M4fXytMP0_-m0A4&rlz=1C1QJDB_enCA643CA644

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check for vacuum leak for the idle or idle mixture screws settings
no power in 3rd gear can be low fuel level, filter problems , ignition timing ( static is 12 degrees BTDC idle )
more than likely a vacuum hose missing after assembly

The simple answer is yes! To both! A blown head gasket will cause a lack of compression and either a black or white exhaust emmision. Improper intake manifold seal will cause rough idle or high idle.

There are a few internet forums that specialize in 60's and 70's Chevy cars and trucks. I would sign up to one of these and pose your question there with all the details you have on the vehicle including those details on the build plate in the engine bay if it has a plate. I'm sure one of the guys there can tell you if you give them all the details you have on the vehicle.

Unless this truck was designed different from all the rest? It should be has simple as pulling off the tire and tapping around the back edge of the drum and it should pop off. Be sure emergency break is off, because that will spread the shoes making the drum wedged on. You did say drum, correct? Because if you have rear disc breaks? Pull tire and remove housing (two bolts) holding the break pads. You may have to stick a pry bar between the rotor & caliper to depress the caliper piston/s. A 350 truck probably has 2-pistons on the caliper. Also. get a couple finger visegrips to depress the pistons all the way down before trying to reinstall the break-pads. This way you don't have to bleed the breaks when you are done. just remember to pump them up a few times before you take off to drive. they will come right back up and should work perfectly.

On the brake drum backing plate under the truck on the rear, you will see a slot near the bottom that has a rubber plug in it. Remove the rubber plug and use a brake shoe adjustment tool to adjust the brake shoes in. This will allow you to remove the drum. The tool looks like this and can be bought at any parts store.

Can you tell me which type of axle you have?

Full Floating Axles To remove the drums from full floating rear axles, use the Axle Shaft Removal & Installation procedure in Section 7. Full floating rear axles can readily be identified by the bearing housing protruding through the center of the wheel.


Semi-Floating Axles

1. Raise and support the rear end on jack stands.
   
2. Remove the wheel.
   
3. Pull the drum from the brake assembly. If the brake drums have been scored from worn linings, the brake adjuster must be backed off so that the brake shoes will retract from the drum. The adjuster can be backed off by inserting a brake adjusting tool through the access hole provided. In some cases the access hole is provided in the brake drum. A metal cover plate is over the hole. This may be removed by using a hammer and chisel.
   

You may have to back off the automatic adjuster before you can pull the drum from the brake assembly

To install, reverse the removal procedure.

You need to de-adjust your rear brakes and/or slacken off the park brake cable to remove the drum as the chances are the drum has a wear lip that the shoes are getting hung up on. ( first make sure there isn't a little retaining screw holding the drum on !..visible with the wheel removed)