My hunch is that someone had a nice set of 17" rims and tires off an AD truck and swapped them on to your '56. I'd stick with them unless you have a problem with that.
There was an earlier comment questioning if 19.5's would fit or not. I run factory 19.5" rims with 8R19.5 Bridgestone R187 radials on my '57 3600. I don't have any clearance issues. I like those tires because they have the tall, skinny stance of the original bias plies and the tread looks very similar. The tread is more of a steer tire than a drive tire, but I don't need a traction tread where I live in Arizona.
I like the 19.5's and highly recommend them, but the factory rims are VERY hard to find and pricey when you do. Seems like even the factory 17.5's are getting harder to find than they used to be.
I posted a couple pictures so that you can see the clearance in the wheel wells and the original look of the skinny tires.
Since you have a nice set of factory 17's, even if they come from another year, and two good front tires, I'd just stick with your 17's. You've got a great price on powder coating. At least that's what I'd do. And then keep your eyes open just in case a nice set of 19.5's at a good price just happens to jump out of the bushes and bite you down the road. It never hurts,,,and is kinda fun...to always be on the prowl for an elusive part or two. Part of the hobby.
Matt
PS--The rims on the TF trucks were originally painted black from the factory. I went with white when I had mine powder coated because I (meaning my wife) liked the look better. If you want yours to look more stock on your '56, go with black when you get them powder coated.
