now that write up makes no sense, first it says the vent plug must be installed before starting, then later after it says to start it, it states "on the 3.8 install the vent plug. I think I am going to go with what bradone said and lift the front end and leave the plug in and cap off and let her run like that and hope for the best.
The method I gave you was learned working in a radiator shop 30+years ago. We used to get $45 for trouble shooting cooling problems but they had to leave the car. As soon as they left I was sent out to "Troubleshoot" with a pressure tester, two jackstands and a floorjack. Get the cap off while still hot. Put on the pressure tester and pumpup to 16lbs. Jack the front up and get it on stands. If it was still holding pressure when done I would leave it for 30 minutes and then go back out with a drain pan (1/3 of 55 gal barrel) and put it under the cap area, confirm the system was still holding pressure, release and remove the tester, start the car, turn on the ac if it had it, and walk away. As an apprentice I always had the dirty end of jobs lined up and learned to listen for the "burping" while doing other tasks. The car would start to "burp", I'd look at my watch and time 5mins, and then go back over, jack the car backup while running, remove the jackstands, confirm burping had stopped, pull out the drain pan, and lower while still running. If no more sounds, top off the radiator and shutoff, re-apply the pressure tester, and walk away for another 30 minutes, then re-confirm no leaks, wipe any excess, and I was done. Total labor time, maybe 15mins. I got 15% ($6.75) for my time. Damn good in 1973 in Florida. Most of our customers were local service stations who couldn't figure out what was wrong. I loved it. They made me business cards and I spread them out all over the county. System works. Shop made a killing, I made good money.
I have never had this fail as long as you confirm no other problems. Use a pressure tester, and not just for 5 minutes.