Rebuilt 318 Refuses to Start

I'd not use a 1/2 cup of fuel..... too much and it will wash the oil off of the cylinderwalls. At this point, the suggestion of a bit of oil in each cylinders seems wise; the pumping and fuel in the intake means you may have washed down the cylinder walls already.

4 to 5 months old fuel is OLD with the new fuels. We have had starting issues before with fuel of that age, and put in some fresh and ZOOOM! It is several factors that effect fuel volitility and it sounds like you may have a large cam and you are going to need all the fuel volitility and compression you can get to fire it off. Putting in 2 fresh gallons won't do it.

And if you did not clean the carb out 100% of fuel when you pulled it, including pumping all of the fuel out of the accelerator pump system, then odds are darned good that some of the ethanol has separated and absorbed water and formed gum. We had this exact thing happen in the last year and it had gummed up the check valve just below the accelerator nozzle in just under 3 months time. It had to be cleaned out. (And then I unknowingly dropped the check valve down the carb.... oops, that's another adventure LOL)

And on the starting fluid.... that is hard on the cylinder wall oil layer and I never use it anymore unless on a lawnmower. But you think it would have at least fired... which goes back to the spark or a big vacuum leak.