"cant i just cut off the spring mounts off of my old axle and weld them onto the new one?"
NO! The spring mounts you have are inadequate for an open driveline. You could get Mopar spring pads at your flaps for less than $30 a pair to weld on to the donor axle. Many of the parts suppliers for our trucks have a rear axle installation kit that usually comes with spring perches and u bolts. You will have to check closely to see if they also come with the shock mount.
Many different rears have been used, there isn't any that are really favored. If you want 5 lugs, maybe try a camaro rear. I don't see how 5 lug would be an update, and modern wheels are ugly as hell! But hey, if that's what you want... go for it!
