Originally, (ast least on half ton springs) the fronts have a threaded bushing pressed into the eye. You can easily press this out and install a new one in a large bench vise. Screw your old pin into the bushing part way to press it out.
The rear eyes are "threaded" when you screw in your new shackle bushings. The bushings are very hard material and will cut the steel of the spring as they go in. You'll probably have to screw them in part way, then back out, several times, each time getting in further. I had a tough time with this but at least I know they are in there tight! They must be extremely snug so they don't rotate. Same goes for the frame bracket.
Check out Rare Parts for new shackles - very high quality and American made.
EDIT: I see the NAPA link has these in their "Rare Parts Chassis" line, so looks like RP now makes them for NAPA. Nice.