Hey Glenn, Great questions, and I'll address them all.
First of all, thanks for the Kudo's.. I always appreciate it. It's still a work in progress, though not much progress lately. I just haven't touched it, though I have been planning some new things. Paint and a home-made canopy are in the planning stages.
As for your questions, first being the metal strips between the bed wood. Simple solution, but a bit expensive. LMC catalog.
http://www.lmctruck.com/icatalog/cbe/full.aspx?Page=20
You will need to know exactly how long of strips you need, as there were several different bed lengths between 1955 and 1966. LMC stocks all of them though. Good quality, and in my opinion, worth every penny. I'm not wealthy, but I splurged on this and haven't regretted it yet. If your bed wood is completely rotted away (i've seen it before) then you may need this website to cut new boards. I cut all new pieces for my bed floor out of pine. I decided to go with the cheap stuff for my first try, and now that I've learned some tricks and lessons, I'll be replacing both my truck and trailer bed floors to match. Should look sweet.
http://www.gmcpauls.com/47-72_BedWood_Info.htm
As for the bigger tires. Well, It sure makes it look like it means business, but honestly, with tires bigger than what's on my truck, it's just plain overkill. I wouldn't say I'm unhappy with them, but I haven't worked out all the bugs yet either. I went with the same wheels (and lug pattern) that are on my truck in case I ever had a blowout and had to do a tire swap to get me into the next town. Hopefully that never happens, but I figure it's planning ahead.
The swaying hasn't been addressed yet, though I just need to lengthen that sway bar underneath the trailer and get it hooked up. Considering dropping the attach point like suggested for a better fit. Piecing that together in my mind, I think it would eliminate my swaying issue. If not, I'll go back to the drawing board.
Hopefully, I'll have some kind of updates by the end of the year, but I'm leaving the country for a month in a couple weeks, so the trailer will have to wait for my return.