Almost there need some help!

[Mr.Hyde]

Ok so here's the deal I am almost "done" with this project truck and I'm aiming for inspection by August 1st. These are the issues in dealing with.. 1968 lwb 283 3 speed column shift.

1. When I apply the brakes with the side markers on, my brake lights turn off. When I apply the brakes with the side markers off my brake lights work like normal. I bought used housing for the rear lights could it be that they aren't grounding to the body?

2. My reverse lights aren't working at all. I have the switch with no nsw and thought that Tim cap was the issue well turns out it isn't. I tried with a new switch leaving it in the on position and nothing. The first lead does have 12v switched power. And these were used aswell. I've looked over everything and it all seems to be wired to the diagram unless I'm missing something.

3. For the bed rest cross member lwb is the rear crossmember welded to the bed sides? Or is it held by the brackets welded inside the bed and underneath?

4. The truck seems to start but once it's left to idle it dies without giving it gas. Not sure if this is a timing issue or the carburetor is just unadjusted.

I know that this is wiring oriented but I figured it's a mix of questions and if get a faster response here move if you wish

[Baddflash]

The first place to start with taillight issues is the ground from the chassis to the box itself, I don't believe there was a lead from factory but connecting a wire from the frame to the rear cross sill will ensure you have ground. I would then recheck what issues remain, backup lights included.
The rear cross sill is both bolted and tack welded in a couple spots. It bolts to the bedsides and frame.
If the motor is running rich it will load up and die, many things could cause this but most likely carb related.

[Mr.Hyde]

Hmm for starting it basically turns over then dies, if I keep it at 1000 rpm she stays if I let it drop it cuts out. I'm thinking the light housings need to be cleaned up because of ground to bed. What should I run for a ground just battery battery cable thickness or 18g?

[Baddflash]

For the taillights a meter can help you confirm if you've got a ground or not, but to run a wire I used a braided copper cable similar to the ones in the engine bay and grounding the cab to the chassis. I would recommend 10-12ga. I also have run an extra ground from the battery to the frame as I do with all my vehicles.
I would increase the idle till it holds and drop it down to see how far you can get before it dies. Hard to really help here as I've no idea on the status of the truck if it runs great or is barely running.

[Mr.Hyde]

It's get to about 500rpm and sound pretty rough, it's a freshly rebuilt motor and when it's running about 1000 it's runs perfectly. It actually didn't start doing this until I remove the plug for vacuum advance and installed a line from carb to distributor.

[Jurassic-1]

Do you have the vacuum advance on the ported vacuum side? it should be ported not constant vacuum. You should also check and set timing at 8-10 degrees. Set your carb with a vacuum gauge if you have one. Typically the idle mixture screws are set about 1 1/2 turn out.

[Mr.Hyde]

Where are the two screws to adjust I see one on the driver side is the second below the choke?

[Mr.Hyde]

So fixed the lights and electrical, all lights work now but it seems like the reverse light doesn't reach all the way like I need to hold it up so I guess I need to file the slotted holes a bit for more adjustment. I'm in the middle of bleeding the brakes now and the 2 adjustment screw on my quadrajet are odd they aren't a slot or hex screw but instead a reverse slot head screw driver I guess? I have yet to adjust the screws out to set the mixture and timing. Getting there!