I just accuired a '68 Chevelle Malibu which original had a 307 engine. Previous owner put in a 350 enigne and transmission from an 83 chev pickup but didn't finish the installation. I don't like this combination, and want to scrap it. Headers, drive line, componet mounts, and electrical problems. (no wonder he didn't finish it)
I got my eye on a 400 hp 396. What kind of expectations should I have for a small block to big block conversion? as far as chassis modification.

None at all.

Just bolt in and go. I have run both over the last year.

That's the nice thing about it, you can have a 383 in your car one week and switch to a 502 another week and it literally is a basic bolt-in swap. :D

Now is that the same for all other years?

Just about any car that has a smallblock in it,,,a big block will drop right in,,,cept for maybe the early Nova years. And those headers are high dollar for a G-body swap.