As Adam said... they are all bolt on and, they can be easily reversed if you want later.
If your trying to keep it to a minimum, and you still have a single exhaust, that is the first place to start, just make it duel.
The second thing I would do is, just bolt on a 4 barrel intake and carb. These two changes will make a difference and, as you put it... not major modification.
Then drive it a while and decide if that's enough power or if you want to go with cam or even heads.
I don't know if you would consider this a major mod but, most cars with 318 came with 2.76 rear gear and, a change to something like 3.23 gear will make a BIG difference in performance Let us know what you decide.
just my 2 cents