What a nightmare. Could you tell us what retainers you are using so this will not happen to any of us?

Is this thing building crankcase pressure? Too many leaks for a fresh build. Seems like you were meticulous on assembly.
Do you have the correct size and type of PCV and tubing and or breathers?  An obstructed PCV tube caused multiple oil leaks
on a Windsor I once owned.

Does your Mustang have the factory tach?  The coil positive wire runs thru the tach to a resistor wire to deliver reduced voltage to the coil. Petronics needs full 12 volts to operate correctly.  Full 12 volts to a stock coil will overheat and possibly ruin the coil.
Make up a jumper with alligator clips and go for a ride. When it acts up, pop the hood and connect your jumper from battery
to coil positive side. This will bypass fuse panel, key switch, tach, connectors, or any Warshawsky  items that may have been hooked to the
key switch circuit over the years.

Look into electric power steering. Several good youtube videos using Saturn, Prius and several other makes parts. It all
fits under the dash and won't leak a drop. If it quits working you keep going with manual steering. Uses a direct connection to steering box so no lash or lost motion.

Your new push rods might be slightly larger diameter and hitting the rocker at full lift.

       Is this the gasket you guys are talking about?       

                                     https://www.amazon.com/MAHLE-Original-MS15166X-Engine-Manifold/dp/B000CLIIEW

Better check the fan. Might be ready to sling a blade.

Check end play. Sometimes yoke will get bent on disassembly or axle yoke will have rust build up or galling on the ears
and will pinch the caps together too tight. Will take out a new u joint fast. Before installing joint on shaft install it in the
axle yoke and check for free movement. If too tight in shaft you might have to spread the yoke slightly with a drift to
get free movement.