Rdin

I have a 1976 Toyota SR5, it is a used truck and has been worked on by many, including myself.
The problem I have is that there is no spark (no spark from the coil) when cranking the engine.
There are many steps between the initial breakdown of the vehicle and my breakdown and asking for help on a forum.
Initially, running at idle vehicle died and would not restart.
Check for spark at the plug no spark, check coil and no spark. As I went through the troubleshooting process, everything was tested before being replaced. Some things were replaced because that was the something I haven't tried.
coil replaced, no spark.
igniter replaced, no spark. replaced with GM HEI 4pin
Distributor replaced, no spark.
When I pulled the distributor (Distributor does rotate with engine, timing chain intact) I rotated the distributor shaft by hand spark, spark, spark, and spark. Install in the engine, no spark. While rotating the distributor by hand I did ground it to the block to see if that made a difference. Nope, no difference.
I jumpered, i ran extra grounds, I cleaned grounds. I ran directly to battery. I bypassed ignition switch. I have done a lot. Everything I could think of, evereything i could find on the internet. I checked any forum Toyota, Ford, Dodge, Reo, Bricklin, anything that popped up in my searches. I don't consider this problem to be brand specific.
I bought an MSD 6AL, I removed everything I originally had and temporarily hooked up the MSD. it fired right up, and I do mean right now. I Then took the time to mount the oversized box under the hood, wire everything. Then spent a day messing aroung with timeing and spark plug gap Blah Blah Blah.
It ran fine for a week and on the Eighth day it did not start.
Now it is doing exactly what it did before. Everything checks out, spark when distributor is rotated by hand, but absolutley refuses to provide a spark when cranking.
The MSD Led indicates Normal.
I say I checked everything, but I didn't. I'm missing something somewhere, I don't what.
I look forward to hearing suggestions.

The truck came to my driveway with
20R, with mag pick up and igniter that appeared to be factory, I don't think the engine is original. I don't know, but if you look at hardware all over engine, motor mounts, transmission, they all bear the signs of ill-fitting wrenches.
Manual 4 speed
Weber Carb, not sure what model
Electric Fuel Pump mount in engine bay? pretty nice.

Punjab

Sorry I'm not much help here. I had some ideas until you said the same thing is happening on the MSD setup. It definitely sounds like an electrical short or faulty coil/igniter but I'm just guessing.

Is the harness relatively in order or does it look like aftermarket junk has been getting spliced into and out of it for 40 years?

If the MSD 6AL fixed it and it really did run great, and now it's happening again...then it could be that the MSD failed somehow. Could there be a broken wire or bad connector pin in the system?

Rdin

The harness is mostly intact. I pulled the tape from the harness on everything under the hood, and most of the harness in the cab. I found a few splices, fixed and and taped what needed it.
The MSD indicates that it is working, it has an LED light that tells that it is powered, and it will flah when it fires also.
When the distributor is out of the block it works, but not in the block.
I'm not ruling out any thing.
Prior to the MSD purchase, while troubleshooting the HEI 4 pin, I wired the ignition direct to the battery, the fuel pump direct and triggered the starter under the hood. The reason for doing it that way is to hopefully isolate the ignition form any problems that could exist in the vehicles wiring harness. It still didn't fire.
I have looked for relays that may cause a no start issue, maybe a low oil level cutout, neutral safety switch, clutch pedal, anything. I just don't see it.
It doesn't sound right but it's happening. If the shoe was on the other foot and someone described this problem to me, I would think they are nuts.
Thanks for the reponse and the suggestions.