Hello, welcome to JustAnswer.
With no timing marks showing, you either have the distributor in wrong, and it sounds like that has been considered and checked, or you didn't get the cam to crank timing correct.
Do you have an adjustable timing light you can use? One that allows you to advance when the light flashes? If so, use it to get the mark to show up where it's supposed to be and then read the advance dial to see how far off it is.
Otherwise, pull the cover and check the timing marks on the cam and crank gears for alignment.
Well, you are going to need a timing light to set your timing on this one.
We also need a fuel pressure reading, compression check, and to check to make sure spark is still there. Check the coil primary resistance and secondary resistance. What are the readings?
Which distributor do you have? What is the point gap set at? Where is the dwell set? Where are you setting the timing?
Ok, the timing should be set a 4 degrees BTDC, But that's not an issue now.
There is just no way for your timing marks to be that far off with the engine running unless you have the timing light hooked to the wrong spark plug wire, the distributor is not installed correctly, the valve timing is off, or you sheared off the key on the harmonic balancer.
I didn't mean to imply you lacked the know how, Andy, It's obvious you know what you are doing. My apologies if it sounded that way.
Check the harmonic balancer. If everything else is all right, the outer ring of the balancer must have shifted on the inner hub. That would explain the engines not running well, too.