My 428 cougar developed a rear main leak, I took the carb off and made a blank off plate with a little 1/4" barb fitting, plugged all the external crankcase openings, dip stick tube, valve cover holes and manifold holes, used a squeeze bulb and by hand pumped a positive pressure into crankcase and could distinctly hear gurgling from the rear main seal, I haven't fixed it yet but hope to do it without pulling the motor, should be easier in a mustang cougar with the removable crossmember under the pan.

I think you cam is back against the rear cam bore plug, put the bolt in and draw it up and the pin will probably end up protruding 1/8" proud is all that's required.

Thanks for all the words of encouragement, something trivial as saddle webbing in a plain jane 390, the shop owner (expert fe guy) said
"surprised there's no crows foot in this one"

I thought about those words all the way home, what was he talking about this was a plain ol block not an early solid lifter PI or HP block, turns out he knew exactly what he was looking at, glad I didn't question him and make a fool our myself.

Feeling pretty stupid I just cashed out >2000 bucks on a 390.
Align bore, decked .005 below deck, .030 over forged pistons, rods re-sized ARP bolts.
Now the punch line, it's a garden variety cast in 69 block, it wasn't until just recently I learned that
390 blocks with reinforced webbing at the main saddles could be found rather easily, I feel dumb because I thought the beefed web blocks were exclusive to 427 blocks, CJ's and never gave it another thought, oh well I'm still confident in it being a good engine for my street driven fairlane, old school solid lifters cam and a 4 speed, nothing serious, 375hp maybe with the headers and R heads exhaust ports opened cj valves PI intake 735cfm cj carb, hot ignition good timing curve.

What pistons are you planning, curious as to what size bore you end up with after cylinders get restored straight and round.
I also like to try and keep the bore standard too if all possible.