slant six starting problems

How, EXACTLY are you checking spark?

What does this spark look like?

You must realize that the ignition switch, and depending on how you do this, can give you misleading results.

If example you have a cranking circuit problem, but you check for spark by jumpering across the starter relay, that there is a problem.

"Rig" a spark tester where you can see it and check for spark USING THE KEY. Do not use a resistor coil wire. You should get a nice fat rythmic spark, nice and blue, at least 3/8" and more like 1/2" long.

What shape is this engine in? Lots of miles? Good compression? You checked the plugs?

Check the cap / rotor, for grease, dirt, moisture, damage, and spark "punch through" on the rotor. Check resistance of the coil wire, at least. Check the plugs, condition?

Has anyone been "helping" with what's in the fuel tank?