Paul,
Synchros for the smooth case are not typically replaceable given the processes required to do so. If yo have a Bentley manual, those things are outlined. I would inspect condition of those before investing any money, If those are good, I think bearings and thrust washers would be the next order of business. I believe there is a bushing in the tailshaft the should be inspected for wear also. I've rebuild numerous rib case boxes, but I've only ever rebuilt one smoothcase ( a long, long time ago), so my practical experience is limited. I've never done it myself, so I can't advise on this point, but I understand rib case internals can be installed in a smooth case if you goal is to appear original.
Regarding gaskets, I don't believe sealer was used when building these originally and I really don't like silicone (overkill, if you ask me). If you want to use something, Hylomar is a much better option. Less chance of gobs breaking free and getting in the working bits. It also makes removal of the gaskets easier and reusable if you have to open it again.
I have one or two smoothcase transmission that I believe are in good condition if you run into a disaster for some reason.