If you haven't messed with timing already I wouldn't suspect that being the difference between running one day and not the next.
Do check for spark at a spark plug wire.
I'd guess (without seeing the truck) at this point that insufficient current/voltage is available to the ignition while cranking. You did replace battery cables, but what size, are connections bright clean metal, and where are they connected, the ground in particular?
Best to run ground cable direct to one of the two starter mounting bolts unless you're dead set on being 100% original and if so this would be a good place to compromise at 99.5%.
I recommend 2/0 cable for these trucks and 6V. If you have 1/0 that should be fine, but 2/0 is a nice amount of overkill without being ridiculous.
If the above is the issue it should roll start nicely as is.
