They could be original and still be power disc brakes. The Treadlevac is different than modern power disc brakes, you wouldn't see the big canister on top. The pedal would usually say "Easamatic" but it could have been replaced. The pedal operates from a pivot above the pedal, and pushes straight into the treadlevac behind the toe plate. If it's manual brakes, then IIRC the pedal is pivoted from the floor, and the master cylinder is under the floor.

One thing that could be true is you could have poor vacuum or a restricted vacuum line, or something else with the vacuum system feeding the brakes, such that the initial push uses up the reserve vacuum in the tank under the fender, but because of the restriction you cannot adequately provide vacuum afterwards.

For reference, here is a thread on the oil fill tube

Oil Fill Tube Orientation

I actually don't know if that could be a problem, but my guess would be a bad lifter. Partly just replying so this could be bumped to the top.

Here is a thread, with a link to some more threads, on Valve Guides removal replacement.

Valve Guide Removal Thread

I don't know whether Packard Don has any special tricks, but I don't think the C-Clamp style works in the car unless you take the manifolds off. I have one of both kinds, I used the C-Clamp style when I had the engine out of the car with the manifolds off, and I haven't been able to figure out how to use the C-Clamp style without taking the manifolds off. This style works fine.

yes definitely. Looks just like mine.

Vendors being Max Merritt and Kanter

I have what I think is the last one he made.

The program you have is probably for an OHV car. Remember that this is a side-valve, there is all that volume over where the valves are, which will make calculations using the head gasket thickness and the deck heiight different than for an OHV car. Also because the Packard valves are canted slightly, the calculation for the volume in the side-valve pocket is not straightforward.

Still seems like would be good to get a different head.

According to the chart, a 1951/52 288 AT head would give you a 9.10 CR, which is quite high.

Some folks have suggested using 2 head gaskets to drop down the CR.

Something that is (surprisingly) simple to do and may shed light on the situation is to run the engine with the valve covers off (no worries, it doesn't make a mess!) and then listen to the lifters via a stethoscope. I found to really figure out which one(s) (though admittedly this is difficult and by no means foolproof) one does best if one puts the stethoscope directly on the plunger/lifter, noting that it moves up and down a bit.

Ross has another video where he shows adjusting the valves for an engine without hydraulic valve lifters which gives one an idea what it's like to run the engine that way.

Speedwell Garage - adjusting solid-lifter valves on a postwar straight-8