1974 D300 360 fresh rebuild will not start

Is it possible you got so much fuel in the cylinders that you washed the oil out?

and above, compression fuel and spark.

Throw a bit of oil into each hole. Not too much, 'about' a teaspoon. Re-run leakdown.

Double check timing. You can do that in about seconds of starter time with a timing light. You want some advance, depending on cam. Maybe, 8-10 BTC with a stocker, 12-15 or more with a mild to hot cam, and at least 15BTC with a wild cam

Make CERTAIN fuel is fressh. If it's been sitting "awhile" rig it up with freshe fuel, IE prime the carb with a bulb squrit a bit down the carb throats, FRESH

If nothing CHECK SPARK. "Rig" a solid core wire, not radio suppression, off the coil to a gap to ground. Crank engine USING THE KEY. You should have at least 3/8" hot blue spark, and likely more like 1/2"