just thinking of your hei, not saying it's the issue since you are saying the thing has spark, when you changed the dizzy did you also change the cap, coil etc or re-use the old parts? if not check to ensure you have a ground wire inside the top where the coil is mounted. I have seen them where there is no ground wire and they have some of the same symptoms. when in there check the coil as well, sometimes they overheat and you will see the discoloured areas on the sides of the coil. like somebody else said, check for a good hot spark at the plug wire.
from another site talking about hei coil failure caused by incorrect assembly of coil-cap-contact etc
The correct sequence is: cap, carbon brush with spring positioned upwards, then silicone heat transfer grease on both sides of the insulator, installed onto the spring, then, coil. If this method isn't done correctly, the distance from the end of the carbon brush to the rotor connector bar becomes excessive, resistance on the secondary side and primary side of the coil rises, heat becomes too high, and the module fails.
a pic of the coil ground strap under the coil and also the coil ground wire that comes out of the coil, see post 10
https://www.chevelles.com/threads/he...ement.1112544/
a link to diagnosing hei ignition for anybody interested
https://www.gmcmi.com/wp-content/upl...stributors.pdf
after trying to start it have you pulled a plug to see if it is wet? does it chuggle at any time when trying to start it or it simply cranks and cranks with no attempt? I would lean towards a fuel problem then if the tune up stuff and timing chain slack seem ok.
just spit balling and info for a possible lurker with the same issue. when you get back at it let us know what you find out for test results. like mr48 says, don't throw parts at it until you know what it needs.