Going thru the 55 cluster isn’t all that difficult, aside from getting it out. The 55 6 volt gauges aren’t that hard to get, the Tbird suppliers have them. Not cheap, but cheaper than a conversion. Or the 55 6 volts can use the 57 and up voltage drop device if the wagon has been converted to 12 volts.
You have to do some digging (I may have it someplace) but those old King Seeley gauges are adjustable, but best done on a work bench. Bit touchy, but doable.
The 56 gauges (or 57 for that matter) aren’t any better IMHO. They are better looking. I’d have put a 57 set up in my bird, but it was just too much work and money. Because of the clock/tach, the dash needs changing and I couldn’t find one.
If you need help with the gauges, start a post. The oil is just a light, if it’s not working it’s either wiring or the sender. The gas gauge should be pretty simple to get close, and I never trust an old one below 1/4 tank anyway. (Actually I never trust any of them. I picked up a brand new car from a dealer last week, and it read full. I got 2 gallons in before it shut off). Likewise the temp gauge. With a digital thermometer you know what it means for what’s normal, so you only need to get concerned when it warming up or goes over. On mine, it’s pegged before the car boils, so it isn’t that useful when working.
As for those who don’t like the *** tra-Dial, do you know how much those fancy heads up displays cost in new cars? Hell, my bird has one every time the sun comes out or I go under a street light in the dark. Ahead of his time Henry was.