First time engine build won't fire

If the D is O-ringed and/or gasketed, and sitting flat it shouldn't be leaking. Weeping maybe. So if you don't find the problem there,then;
pop your CC breather out and make sure it is venting in BOTH directions. In order for the D to be weeping, CC pressure has to force oil up past the top bushing. If you have this much CC pressure,the breather should be flooding the top of your valve cover.
You can test the CC pressure with a low-pressure gauge; just flip the PCV out of the grommet, and remove the breather, and seal the two holes. Then plumb your gauge to the dipstick tube, and start her up to idle only. Watch the gauge and time how long it takes to hit 3 to no more than 4psi. Do not let the pressure increase past 4psi, as that would be risking blowing out a seal or gasket. While it's building, you can go check the D for leakage. I use a spritzer bottle with soapy water; it easily finds the leaks.
Unfortunately I don't recall how long it took for mine to hit 4psi, but it was quite a while.I'm gonna guess a minute and a half to two minutes would be ok. If like 30 seconds,then you have a problem!and it's time for a leakdown test. or second best, a compression test.
Happy hunting.