The Kingpin chassis had better handling than the ball joint setup that replaced it. We had a 1983 E350, kingpin axle, and like you, I replaced the steering box and shored up the rear springs.
I did several other things.
1. Replaced the Axle-to-Frame Bushings. I've seen them really shot, ours weren't too bad but I'm sure new ones helped. I was able to press them in/out with a borrowed Ball Joint Press kit.
2. Replaced the Radius-Rod-to-Frame Bushings. Easy job, and bad ones all allow the front end to toe OUT, which you don't want.
3. Replaced Front Sway Bar End Link Bushings. They were OEM rubber, and softened. I installed a poly bushing kit from NAPA and the change was amazing. This was really a big hitter in the improvement lineup.
4. Adjusted Toe to slightly IN. There are various DIY ways to check and change Front End Alignment, and TOE is the simplest. With some Toe IN, about 1/4" the way I measured it, the driving really became great. WAY better than the E450 in my signature, and the E450 has Sway Bars, Air Bags, Track Bar, and offset bushings to increase Caster.
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB