Hi, That's a nice looking '56. I had a similar one back in the 60's. Regarding plugs, with a basically stock Y-block there is no particular reason to deviate from the stock Autolites. I can't speak for the others on this site as to what they are using, hopefully some of them will chime in. If you have a favorite brand, then give them a try. Here are some part numbers that will fit your engine:
Autolite: 46/ BSF82C Bosch: 7551/ DR10BC Champion: 21/ F14YC Motorcraft: 30/ BF82C (old p/n) 631/ BSF82CF6 (newer p/n) NGK: 2438/ WR5
The first number is the stock number used at the auto parts stores, the second is the actual plug number.
I would open up the gap to .040" or .045" if you are running a sold state ignition module. A longer spark will help somewhat. I also believe in "indexing" the plugs, that's where you orient the gap so that the incoming air/fuel mixture sees the open gap, rather than the back side of the ground electrode. I think Ted is going to do an article about this in Y-Block Magazine. Good for a few HP at high RPM.
All of the plugs I've listed are fairly hot in heat range, for performance use and to reduce detonation I'd go a heat range colder.
Have fun, Nick Brann - K.C., MO