I would go through the 350. If you aren't looking strictly for performance it is more cost effective. You can do a good rebuild with a kit from Nortern Auto Parts... http://www.northernautoparts.com/Pro...uctModelId=380
I doubt you would have to replace the rods so this kit will work for you. They have other more expensive kits so look around on their site.
What I would replace is the TH350 trans. Lose it. Put in a 700R4 rebuilt to Corvette specs and will you see a major increase in mileage, more response off the line and less engine wear at highway speeds. Look here... http://www.smokemup.com/tech/700r4.php
Lots of good info. Run through the RPM calculaters to see the benefits of the O.D. swap... Best thing you can do.