Just to jump on Kevin, Ed and Otto's bandwagon ...
Replacing/upgrading your front end because you *want* to is one thing. And if that's the reason, more power to you and we'll help all we can.
Replacing/upgrading your front end because you think you *need* to is entirely another. And for all the reasons stated above. I'll just add that from pickups to tractor trailer trucks, I've put a fair amount of highway and back road miles in on straight axle trucks. The truck I drive for work most days has a straight axle (but with 11R22.5 tires, I don't mind the power steering ...;) ) and I hit plenty of pot holes around the Nation's Capitol ... I have no problem or hesitation to take my unrestored (but well maintained
) '49 1-Ton on the interstate -- and have, many times. It will cruise nicely all day at 65 MPH and sprint to 70 when needed. Straight axle, Huck brakes and bias ply tires. It has a little wander above 60, but its manageable. Potholes? Not too bad. At speeds above 60 MPH, I just try to hold the wheel straight and the truck will quickly resume course just fine after a few milliseconds of hinkyness.
Why is it able to do this? Have I put sway bars on it? Have I upgraded to an IFS? Do I have Power steering?? No to all of those. I am running the truck in its original configuration (suspension wise
I did put a 261 and a 4.10 gear set in the truck) because *everything* is still within spec -- Springs, shackles, steering box, tie rod ends, pitman arm, front alignment, etc, etc. Just *one* of these things out of whack will cause issues at modern highway speeds.
Does my truck drive as well as a modern truck? No, of course not. But it drives fine. It's a different experience and requires a little more attention to what you are doing. Doable? yes. Enjoyable? Certainly.
The '65 GMC 1.5 ton I used to own was the real teller. It was a firetruck and it only had 8,000 miles on it when I bought it in 2007. Mechanically, it was still new. There was zero wear in the front end. You could drive that truck with one finger, even at slow speeds. At 65 MPH (um, I removed the governor and replaced the crappy Stromberg carburetor with a Holly 300), that truck handled like it was on rails!
So ... in my opinion, FWIW ... The factory straight axle/spring suspension/manual steering set up is perfectly adequate for today's highways as the boys have said above ... IF ... and this is a big if ... *everything* in the front end is within spec -- king pins, alignment, tie rod ends, pitman arm, drag link ends, steering box adjustment, springs, spring shackles, spring shackle pins, tires (age, tread, balance and pressure). The more *any* of those are off, the less enjoyable your driving experience will be.
Bottom line -- If you *want* to upgrade the front end, you go right ahead and we'll help you all the way. Just don't think you have to because the straight axle set up is "bad."
If you have a straight axle truck and it handles poorly, it just needs to be fixed, not heavily modified. Not at all against performance upgrades (Evan ... Heck, I'm a Group W Bench Stovebolter myself -- I put a 261 in my '49
... and created a nuisance ... ) -- Just do them because you want to, not because you think you have to.