Ok, this old monster is my dump run/ hunting truck so it doesn't see as much abuse as my wheeler, but I have a 78 chevy w/th350. I would like to start driving it a little more often , but the trany is slipping. If I have to tear it apart I would like to get something with an overdrive.
What are my best options? and how big of mess will it be to do the swap? + the only option that I know of is the 700r4 are there other worthwhile options?
As far as fabrication goes I can do it, but I don't need to make it harder than it has to be.

you would be stepping down in strength if you swapped out a th350 for a 700r4 unless you do some work to the 700. If you are not against a manual trans, you may want to concider a NV4500

I've replaced many 700's with 350's!!
I'd fix it (they can be rebuilt on the Cheap) Or swap for a manual.