Options to keep 3 on tree but upgrade trans

[Moneypit]

I love driving my 68 with the 3 on the tree. It originally came with a 307 and will soon have a new 350 in place. I haven't pulled the trigger on anything yet so I don't have any specs but it won't be anything crazy. The truck also has a basic rearend, 3.08 peg leg I believe.

I figure if I'm going to spend the time and money upgrading the engine I should at least take a look at the rest of the drivetrain and consider my options. The truck won't seen any heavy hauling or racing and I'm not looking to make it anything spectacular. Just something that is a bit more reliable and fun to drive. I really want to keep the 3 on the tree so what are my options for the trans/rearend? Should I have the trans rebuilt even though there doesn't appear to be anything currently wrong with it? If I do get it rebuilt should I stick with the stock gear ratios keeping in mind it will see mostly around town driving but some highway as well? What about beefing it up in anyway and making it stronger, is that even an option? Or should I just leave the trans alone for now and put in some lower gears with a locking diff?

Thanks for any advice.