What is the i.d. on the housing? IIRC the shaft is .312 in diameter.
When I replaced the seal on the 400D I noticed that is was simply two regular seals inside a sleeve. I am wondering if you could retro-fit something similar; two seals in the same housing and lose the packing nut, glands and backer. My book shows both pumps exploded but it's a drawing and non ultra-clear.