The only thing that I definitely would do is add power disc brakes to the front. I don't think the car is a candidate for a clip swap from another car. Like Don said, there are companies making every conceivable part for the car, so if anything, I would just make sure everything in the suspension and steering is tight. Take a look at Master Power Brakes for the front disc conversion. There are other companies as well, but MPB is probably the best known.
www.mpbrakes.com

Here's another, Stainless Steel brakes.
www.ssbrakes.com

If you do go with a stronger 350, the stock radiator will probably work ok if it is in good shape. I would make sure to fit a shroud to the back of it with an 17"-18" steel OEM engine-driven fan and thermostatically controlled fan clutch. The fan blades should be half-in and half-out of the hole in the shroud. Other guys will tell you to run an electric fan, but in my opinion they are garbage. You'll have to decide for yourself.

There are some companies making power steering retrofits with the GM 605 box. Do a search and check it out.

To bring the car back to stock height, you will probably need new front springs. You can get the stockers from Eaton Detroit Spring.
www.eatonsprings.com
While you have the front end disassembled to install the new springs, check all the other components such as control arm bushings, ball joints, tie rod ends, etc. Your local parts source such as NAPA will most likely have parts such as that.

Hope this helps. If you need further help, just ask.