Timing question 318 with LD4B
You can check the timing-chain slack from underneath the vehicle no problem; I have done it several times.
My brother brought me his van of about that vintage, but there was no way to check the timing. So what I did was drill a large hole in the BH, about in line with the ring-gear, and with a piston stop, I determined true TDC of #1 piston and marked the torque convertor thru the hole. Then I could use my dial back timing light on that mark.
In the video, it can be seen that the choke is partially/ mostly closed, and the carb seemed to be on the fast idle cam. If that is true, she may be flooding.
You said;
Once engine is started it will idle and even rev up nice,
In this case I really doubt the chain has jumped.
I tried to listen for a vacuum leak, but that engine seems to be making a lotta top-end noise, which made it impossible to hear any vacuum leak, for me at least. But I also could not see a vacuum-advance hose.
If that van left the factory with a lean-burn system, then is it possible that it still has the ELB distributor in it? If so, it will have to be swapped out, because it has no provision of any kind for providing timing-advance.
If it just quits as the rpm comes down, then I would suspect, in order of probability; stale gas, a huge vacuum leak, a non-functioning slow-speed circuit in the carb, extremely retarded ignition timing, metering rods bent and stuck in the lifted position, or a missing checkvalve in the accelerator pump discharge circuit.