There is a lot of info on this thread:

https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?topic_id=25086&post_id=240608#forumpost240608

You need to change the flasher to a solid state flasher (info in the thread)

For reference here is R&A Engineering's website. At least that is still up:

http://www.ra-restorations.com/index.html

Again, even a "modern" 4- or 5-speed unit will have a 1:1 or perhaps a slight overdrive for top gear. It won't help much with your "too many RPM's at highway speed" issue.

In any case, putting that 4- or 5-speed will be a lot of work, and take away from what your Dad did.

Maybe you want to talk to a Vintage Chevrolet (non-truck) forum. They seem to be a bit more welcoming and also knowledgeable.

https://vccachat.org/ubbthreads.php/topics/395256/all/36-high-speed-diff.html

I use this:

https://redirad.com/

it's simple to install, doesn't destroy your classic radio, but it let's you play all your music (and even get FM) through your phone.

Bars Stop Leak comes in liquid and pellet form. People seem to recommend that.

My favorite sealant for difficult situations is Copper Cote, it's good for up to 400F:

K&W Copper Coat Gasket Compound #401504

This stuff is similar and also good to 400F:

Permatex Aviation Form-A-Gasket-no-3 #80019

These are whitewalls, but not wide whitewalls. Work great on my 1953

https://www.carid.com/hankook-tires/optimo-h724-white-wall-p235-75r15-635263942.html

My understanding is Packard designed and built it's own axles up through 1955.

https://www.macsmotorcitygarage.com/1926-packard-pioneers-the-hypoid-final-drive/

In 1956 Packard introduced the new limited-slip "Twin Traction" differentials. Presumably as a cost-saving measure, all the axles, limited-slip or not, were manufactured by Dana, though the technology it seems was developed by Packard, the patent is 2769353 "DIFFERENTIAL GEARING" - Edward L. Nash, assigned to Studebaker-Packard Corporation, documented in SAE paper 560247 "The Packard non-spinning differential" by Forest McFarland and E.L. Nash.

Kanter has rebuild kits for those carbs for ~$60. They aren't very complicated.

https://www.kanter.com/packard/pac-454.html#9

Those oil-bath air cleaners are I believe very low-restriction. The air just flows over the bath of oil and the particulates are trapped in the oil when they are forced into it by centrifugal effects.

The radio operates off of 6v. Remember, everything Packard (and practically everything from every other manufacturer) operated off of 6v until 1955.

Internally, there are parts of the radio that operate at much higher voltages than 12v, that's what the vibrator does, change DC to AC, then the AC can be run through transformer(s).

I have found that variable power supply is handy to have around for lots of reasons. For instance, I used it to test the electric fans on a 2000 Jetta just this past weekend, and if it's in the garage you can easily test automotive components, bulbs, fans, etc., but you can also pick it up and prop it up into or near the engine compartment or what-have-you, much more convenient than trying to run jumper wires from a/the battery.

I thought your car was 6v?

I used a benchtop power supply like this. They are handy to have around anyhow:

On Amazon for $50:
DC Power Supply Variable,0-30 V / 0-10 A LW-K3010D Adjustable Switching Regulated Power Supply Digital,with Alligator Leads US Power Cord Used for Spectrophotometer and lab Equipment Repair