I only ever had the problem on a motorcycle that had the treads that were radial around the circumference that wanted to follow the slightly zigzag anti-skid cuts that pavement sometimes had in the ‘70s. I drove my 1940 110 all the time at highway speeds and never noticed it but that was with rebuild front suspension and new tires so as others have indicated, looseness is the first thing to check.

As Ross indicated, this information is not needed as it's a simple swap but also just to clarify what he hinted at, the cam must be changed too as the lift is quite different between solid and hydraulic lifters.

The document the BigKev posted will also tell you if your car has the proper engine as it shows the range of numbers for that year and model.

The vacuum tank is supplemental storage for the power brakes and gives one or two power boosts if the engine dies while driving. It gets its vacuum though a one way valve fitting on the manifold which needs to be checked for proper functionality when reassembling everything.

The 298 number is the anti-theft number and could get you through but typically this number isn’t used for registration. What was the 515 number you posted? That looks closer to the type of number that Packard used, at lease on earlier cars.

The vacuum pump was for the wipers which originally had a vacuum motor. If still vacuum, they will work without it but my show down considerably or even stop on acceleration. The distributor's advance still needs vacuum from the intake manifold to work properly.

Very nice, Kevin!

If 12v, then definitely use a 12v solenoid! Otherwise it would likely have a short life with twice the voltage than it was designed to handle. The starter should be rewound for 12v too as the higher impact of the gear to the flywheel could damage one or the other.

If they've been sitting more than a few years it's best to rebuild the cylinders and replace the hoses.

On my former prewar Packards which went back only to 1939, the valve was always threaded directly into the head with the outlet at 90 degrees. I probably even have an original somewhere but it would be at my Oregon shop.