synchro interchange/best ones to use

The late style shifts a little easier. Just a little bit. The brass rings are less likely to split at the strut locator.Otherwise they are exactly the same. For street duty I wouldn't lose any sleep over it.
As to the brass forks; I have seen a few brass forks split off the stumps. I have no idea under what circumstances that might happen, although I have suspicions. It has never happened to me.
As to the steel forks, they are a little harder on the sliders, if you continue to use the forks when clearly the sliders should be changed, due to jumping out of gear.
Again for street duty, I would run whatever you have; just change stuff out when it goes bad, and upgrade then.
I wouldn't change stuff just for the sake of changing stuff.

Yes the O/D forks will work,same as the new stuff.
As to swapping synchronizers; it's an all or nothing deal. All the parts in each style are different from each other;the hubs,sliders,rings,struts,and springs;all different.