Hi Matt,
I just replaced my 235 with a 350. First impressions are that there is a lot more power with no adverse impact to handling. I think that the V8 is lighter than the original engine so your springs will be fine. Relative to boxing the frame, you won't need to if your goal is to maintain the original "feel" of your truck.
Power steering is a nice addition, especially if you run wider tires. Same with disc brakes, assuming you are staying with the straight axle long term. The best change I made to my pickup relative to handling was to add a front sway bar.
Best of luck! Let us know what you decide.
