i think there was 5 ground wires coming off the firewall, id start from the firewall and follow all of them, make sure they connect to something.. maybe pull them back off and clean them up with a wire brush.

did you clean the battery posts/connectors as well

 

Is the all the vac lines connected to the manifold , brake booster ext ?

 

Is it draining the battery? Do the wires that attach to the battery get hot? Put the battery back in the 300, is it still ok in it? You could also test the truck battery in the 300 (or better, bring to an auto parts store and have them do a free test on it). If its not drained, I'd say its a ground or hot wire loose somewhere. You can isolate by using a set of jumper cables. First try the ground (it's safer). Just find a good grouding point on the truck, attach one end of the cable, and then the other ond to the battery. See if it cranks or at least turns. If not try the hot directly to the starter. First, DISCONNECT the hot from the battery so while you are trying to get the jumper on the starter, you dont short to the frame near the connection. Secure the cable and insulate with electric tape so if it moves it won't short out. Then reconnect the battery AND the other end of the jumper. Try to crank.

One last point...don't use the same grounding point thats in questiin.

 

Finally got it!

Battery connection was right... I decided to clean the terminals really good with baking soda/water mixture (they didn't look that bad to begin with).

BUT... after scrubbing on the negative cable.. found there were two sizable cracks in it.

I was able to get it running as-is, but the negative cable terminal will definitely need replaced.

Thanks for everyone's help/advice! I really appreciate it!!