Thanks for all the input.
I'm confident the ground, chassis, and block are all grounded. The negative cable goes from the battery to the block with a smaller negative cable going right to the fender. Then there's a ground strap from the back of the block to the frame and another ground strap using the same bolt on the chassis going up to the cab. We cleaned all the connections as well. Just to make completely sure the cab has good ground, I ran a wire from where the negative cable attaches to the block to the metal bar that goes across underneath the dash. No change with any of my issues. Also, everything else works fine, gauges, radio, cigarette lighter, heater blower motor, etc.
I jumped the ground for the started relay to a good ground with the relay installed and now the truck will crank with the turn of the key. Interestingly enough, with that ground jumper in place, I can also connect to the ECM with my scan tool. Surprisingly, it's not throwing any codes. All the sensors seem to be working, I can read RPM when cranking, throttle position, air flow, etc.
The local O'Riley's had an ECM for a manual 305 in stock so we gave that a try and didn't have any luck. We also tried a known good, used, ECM from another 305 with the same results.
One thing my scanner doesn't seem to be getting a signal from is the ignition timing. Although, I'm not even sure if timing is a parameter that can be read through obd on these trucks. With a timing light, the timing reads dead on TDC. I also double checked the firing order in case you were wondering.
We tried some started fluid just to see if maybe fuel was the issue but it resulted in the same cranking with occasional sputter here or there.
We looked extensively all through the dash for a ground distribution and couldn't find anything. If anyone knows where the starter relay, OBD port, and fuel pump get their ground from, that would help me tremendously. Wiring diagrams aren't too helpful with the locations of grounds because most of them just say that it goes to ground but not where that connection actually is.