Do you mean LS series V8? The best driveline is whatever you're happy with. An LS with a 5 or 6-speed would make a great driver. You could also use an older TBI engine and still have fuel injection for trouble-free driving. Many of the 4.3's are TBI, but you could also get a 305 or 350.
You could swap to a diesel, but it's hard to find someone good to work on a diesel if you have trouble with it, at least compared to a SBC. Go with whatever you're comfortable with.
If you're going with more power, you need to make sure your suspension and brakes are sufficiently upgraded for the higher speed. I assume you're doing this, but don't know what you've done with the chassis.
I'll probably get beat on for saying this on this site, but if you're looking for a reliable long distance driver, upgrade to a newer motor. Not only will it be reliable for longer and require less tinkering, but if you do break down on the road you'll be able to find parts almost anywhere.
