Need help figuring out wiring problem

[GUITAR1989]

Hey guys I finally got my Vortec 350 into my 74 Stepside!!! Only thing is Im having wiring issues. Tach stopped working so I bought a new one. When I was installing it I noticed my gauge lighting wasnt working. Then my starter crapped out on me. Then a speaker...next day the other speaker stopped working and head unit goes off and on. Today my headlights wouldnt come on...Im lost I have a DMM but im not quite sure what to check. My batt is at 11.?? volts when its just sitting there key off but as soon as you put the key in on position voltage drops to about 6 volts. I know the actual wiring pretty well so I can answer anything that you might need to know. Im going to draw up a diagram just so you guys can take a look and tell me if i wired up my engine right. Its carbureted and pretty bare bones under the hood so wiring is honestly pretty basic but i just dont know what to check for.

Ive been doing all the electrical testing with the busted starter since I dont want to put on a new one and have it get fried.

I have an Electric Fuel Pump, Oil Pressure Safety Switch, edlebrock 1406 Electric Choke, Mallory HEI,

A few things im concerned about are the starter wiring and oil pressure safety switch mainly. I read GM HEI equipped cars dont require you to use all starter posts but my starter has wires to all 3 posts.

anything else you need to know just ask 

[bd]

Congrats on getting the engine in!   

      ?
 ?    ?
      ?
But, you'll be hard pressed to test the electrical system with a bad battery.  The battery is the foundation of the entire electrical system.  If the battery won't charge up - and then hold its charge after the ground cable is disconnected - you'll need to replace that battery before continuing. 

It would be prudent to test for static draws on the battery, too, with cables hooked up and e/g turned OFF, doors closed, etc.

[GUITAR1989]

Hmm its actually a pretty new battery. What leads you to the conclusion that its bad?

[bd]

...I have a DMM....  My batt is at 11.?? volts when its just sitting there key off but as soon as you put the key in on position voltage drops to about 6 volts.... 

A fully charged battery is 12.6 volts (with the surface charge removed), and certainly won't drop to 6 volts just by switching the ignition ON.  Of course, from your post I assumed that you used your DMM to measure the voltage and didn't rely on the dash gauge, since the dash gauge shouldn't indicate any voltage with the ignition switched OFF. 

Poor cable connections to the battery could also account for some of your symptoms, as could improper grounds.  Nonetheless, start your diagnoses with the battery and make sure it's okay.

[GUITAR1989]

Hey guys I let my battery charge overnight and took out a bunch of old wiring then ran new grounds...she fired right up. There's a terminal on my fuse box that the voltage just fluctuates with the key off. It won't let me add the pic so I'll post it up when I get on my laptop. It shows the exact terminal in question

[GUITAR1989]

I might have been shaking or knocking the mm ground when I tested that terminal cause I just checked it again and all seemed fine