I believe the 356 is the largest displacement engine this HC head will fit.

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I pulled the 356 head and did some comparative volume measurements of the combustion chambers. The 356 measures 95cc, the 327 head is 90cc, and the HC head is 120cc. Very little difference between the 356 and 327. Why the supposedly HC head is so huge is a mystery. With that head I think the compression would be too low for the gasoline to even combust -- the engine would not start. The plan is to install the 327 head but with such a small difference I'm tempted to check volume on a 288 head to see how it compares. I'm wondering if the 356 head has already been planed at some point in it's life. If one does the math assuming the 356 head is stock, there's a 5% difference in volume so a CR of 6.85 would become 7.21, not the 7.53 per the chart.

However, as it seems whenever something is torn into other issues arise and this time is no different. Head removal revealed a problem with cylinder #4. The rest all look just fine and the head gasket appeared fine with no indication of leakage anywhere. There is oddly some severe metal erosion on the intake valve head and on the block between the cylinder bore and exhaust valve. Picture attached. The seat on the intake looks okay still, but the seat on the exhaust looks compromised -- not severely but I assume there's some leakage.

Any ideas how this could happen on just #4?

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I don't think that's the case for the '55. Here is the underside of the cover. The bottom lip rests on the rubber seal. If the seal is not there it can only rest on the metal lip of the filter.

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It's what the cover rests on. Otherwise it's metal to metal.

Here's a picture of the air cleaner with what's left of the seal/cushion, which is flattened, cracked and partially missing.

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The air cleaner for the single 4bbl V-8's appears to have a rubber O-ring about a 1/4" cross-section around the oil bath body which seals and cushions the interface to the cover. I'm not seeing this seal in the parts book. Does anyone know if these are repo anywhere or perhaps some bulk O-ring material would do the job?

See https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?start=0&topic_id=25666&order=ASC&status=&mode=0

On the upside, with the standard Olds Melling pump with stock relief valve spring (it came with a "high pressure" spring for optional installation), I've got great oil pressure on my formerly rebuilt V-8 and the lifter ticking is now gone. I did not opt for the high pressure spring as I expected it would not increase idle pressure but would boost the higher rpm pressure more than really necessary. I'm observing a steady almost 3/4 scale indicated pressure on the gauge at all rpm.

One surprise when I disconnected the internal vacuum tube to the vacuum booster is that it was mostly full of engine oil. The oil pump bushing appears to be tight, no play. I've not yet disassembled the original pump but am thinking the air ingress is from the vacuum booster rather than the shaft.

Packard for many decades employed a "floating pickup" combined with bypass oil filtration, to provide superior filtration with a single filter. Bypass filters filter out smaller particles than do full flow filters. The floating pickup is intended to pick up the cleanest oil as heavier particles sink, ensuring no large particle gets circulated.

I just replaced my pump several days ago with Jack's Olds kit. In addition to the rigid pickup, I also noticed that the pickup sits higher than the Packard original, like about an inch. I take this to mean you won't want to let your oil level get low or you'll soon be sucking air.

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I have not found a working solution to this arrangement. Using the solid state flasher (which by itself improved brightness) I've tried several different models of equivalent to original polarity-insensitive LED bulbs and they appear to all have crossfeed problems through the lamps with the internal circuitry. For instance, even with just the fronts switched to LED the rear operation is all messed up, with brake, park and turn lamps not operating normally.