About to replace the wood bed, have some questions!

[1963-4x4]

So, I just finished tearing out all the old, rotted plywood from my 63, and I have a few questions regarding replacing it.

I can't afford a kit premade for replacing it, so I'm resorting to some DIY options. I plan on using 2x6's for my replacement wood, and without the metal strips that go between them. I was wondering how much should I plane off of the wood, what kind of gap should I leave between them, and if 2x6's are even the best option. For anyone who has done this before, how wide was the wood you used?

Pictures are always appreciated!