The best bet would be something like an RV cam that will give you a good cruise and great throttle response,

You're not going to be revving it high so something that's around 212I/218E @.050 on 108 in at 104 would certainly make a 318 a bit more snappy,

the '302 heart shaped chamber heads from a late 80's 318 with a little clean-up work in the bowls and a nice valve job would make a big improvement in torque, even with a stock-ish cam,

Even if you do go 3.55 or 3.73 gears you're not going to do a whole lot better than a cam in that range, IMO it's better to slightly undercam a 318 than to overcam it.