Tim,
I went through the same thing recently. Mine is a '70. A lot will depend on how corroded and rusty your lower seal pieces are. You can pick up a replacement seal kit for relatively little money.
In theory, the project is a piece of cake. Search for previous discussions on the procedure. In a nutshell, you remove the lower retaining clip and "blow out" the old seal by starting the engine and turning the steering wheel to the far limits. The pressure should blow out the seals. Then you remove the pitman arm and replace the seals.
I say "in theory" because for me it wasn't so simple. My seals simply wouldn't blow out under pressure. Things were just too crudded up I guess. After hours of trying, I wound up removing the box and after more hours finally got the seals out on my workbench.
As for your question about whether you should rebuild the whole thing or not, it depends on whether or not you want to just repair your leak or dig in deeper. If things go well for you, you shouldn't have to dig into the box.
Good luck. Tom
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TC Member #59
'70 Malibu 350/4speed
'90 John Deere SS165
[This message has been edited by TK-70 (edited 08-20-2001).]