Merritt has them. Part # 12
BCS101, battery box cover screws

The article you probably recall is in the owner's handbook, attached are the relevant pages. There is a copy of this very useful handbook for sale on ebay now, search on "1929 Packard owners Manual"

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The engine speed at 25 MPH would be around 1200 RPM depending on the final drive ratio and wheel size, there are plenty of online sites that have the formula.
The oil pressure will not continue to rise in proportion to engine RPM because there is a pressure relief valve on the pump, If it's set to open at say 25 PSI then it will not rise above that at higher RPM.

If it's the same as the 1922 126/133 models there is a vertical control rod that must be disconnected and the electrical cables connected to the starter switch must also be removed before the floor will come out. It an real PITA job!

I did the same, an inexpensive new coil (plus a spare to carry) and an ignition switch bought from a boat shop. Apart from the slightly different bezel holding the switch, the difference was barely noticeable. The new coil has a slightly smaller diameter compared to the original so a sleeve around it allowed it to be clamped into the holder behind the dash.

New reproductions are available :

https://mykmlifestyle.com/1929_Packard_Ignition_Coil.html

The car in question (1929 Packard) has a vented float bowl so boiling fuel in the carburetor may not pressurize the bowl. The vent is a small hole in a screwed cap, I removed the cap on my cars providing a bigger vent opening, that made no difference
In my opinion the problem is simply that the fuel drawn into the engine is a mixture of liquid and vapor. The vapor has negligible energy so the car starts "bucking", as the vapor part starves the engine of fuel. Pulling out the choke or full throttle can provide slight improvement but once it starts, stopping and cooling the vacuum tank and carburetor with a wet rag will provide temporary relief.
The problem is as simple as that, the cure however is far from simple, how to keep the fuel cool.
It's worth adding that on later model cars vapor lock might be due to the fuel pump not delivering vaporized fuel, pumps deliver liquid not vapor. On cars with a vacuum tank, its not a pump, its simply a tank under negative pressure (when the float drops) and as such will suck liquid or vapor if required.
A final comment despite this being a real PITA when it happens, sometimes a year or two will pass without a vapor lock incident occurring leading to the belief that the latest "fix" has somehow been the cure, then when conditions are "right" it happens again

In discussions with a fuel company (BP) I was told there is no relationship between octane rating and fuel volatility. What you may find is that the higher octane fuel does not have ethanol

I had two 1929 Packards, on one I re-ran the fuel line down the opposite side for the reasons described, it made no difference. There is very little heating of the fuel in the line from the fuel tank to the vacuum tank. The problem starts with the fuel stored in the vacuum tank located in a very hot engine bay and above the exhaust manifold.

When you say that priming the vacuum tank didn't work, do you mean it started but still had no vacuum or it would not start? If it started then there is inlet manifold suction so the problem is between the manifold and the vacuum tank. The suction at the pipe where it connects to the vacuum tank when the engine is running is strong, is the piping blocked?

The Packard company issued this letter to its dealers in 1931 giving instructions about modifications to avoid vapor lock. It states that the fuel had become more susceptible to vapor lock. The fuel today is far more "susceptible to vapor lock" than it was in 1931. The problem is that it vaporizes or boils at a much lower temperature. It is certainly not a new problem

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