I wouldn't necessarily rebuild the engine for an oil leak. Depending on what's leaking it could be a very simple fix. You don't say what engine, but the oil leaks that I see that leave a spot in the driveway are usually oil pan, rear main, or intake gaskets. None of those require a rebuild.
Unless you're going to drive this truck for another 170,000, I probably wouldn't bother with a rebuild, especially if the engine still has good power and is running well.
Now if you want to rebuild it "just because", that's fine too. Cost is going to depend on how worn the engine is, how much machining is required, and what parts need replaced. Most engine shops can give you a good estimate based on their experience from other engines, but there will always be an allowance for parts that turn out to be non-rebuildable once it's torn apart. (bad crank, worn pistons, etc). I had a 390 rebuilt by a local shop a few years back. I had to buy new pistons and a cam in addition to the standard stuff. It ended up costing me about $1300 when it was all said and done.