Others have solved the same problem by adapting a modern master cylinder. True and a viable option. Problem is because of the unit being at the bottom of pedal, there is no modern vacuum style replacement that can use the same 1:1 pedal ratio and will be a bolt-in at the same spot. There are conversions that mount higher on the firewall in the air vent area or even higher which require a different pedal completely. The other conversion will mount in the original spot but also requires a very modified pedal. Any modern vacuum unit needs to get a 3 or 4:1 or higher pedal ratio.
A few people here have done both conversions and believe either is satisfactory. If interior looks is not a problem, probably the most cost effective and easiest way is mounting the unit in the same spot and modifying the pedal which changes the looks, height and feel quite a bit. The only conversion I know of that may work with essentially the stock pedal hasn't been tested yet so currently not a viable option. Another unit (the split component Electro-boost) has also been installed by a couple of people but the units are around $1000 with the installation additional. No feedback posted that I recall seeing on how they are working out. Any of the conversions do have the advantage of, with very easy plumbing changes in the 51-6, having a dual master cylinder split system for safety.
Two places are a possibility for getting the street rod setup. Master Power Brakes
http://www.mpbrakes.com/ or ABS Power brakes
http://www.abspowerbrake.com/ (which developed the Electric boost.) Unfortunately, the frame isn't high enough for a vacuum power unit to fit under the floor.
Here is a link to a thread showing the modified pedal on a 56.
https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=64462#forumpost64462 and another in an earlier car
https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=67023#forumpost67023
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