Thanks guys! You are really helpful.
Don, to answer your question, yes, the cases were annealed. And maybe not as far down as I need to go, because the 3-1/4" case has a surprisingly long neck.
Craig, good points. Maybe I need a forming die here. The one RCBS custom die probably is a forming die, but the press may not be up to the task.
Kutter, good points. Me and cheater bars are not a good mix on big rusty things, so on a reloading press it would be a disaster. The parent case is the ubiquitous .450 3-1/4" NE, and the same shell holder is used for both .450 NE and the subject .450/.400 3-1/4" "Four Hundred Magnum."
Reading all your suggestions makes me think my RCBS Rock Chucker press is probably at maxed out leverage here. The long neck of the 450/400 3-1/4" case is about a quarter inch longer than the press can go. Danged shame. Before Don put me onto the guy selling the 450/400 dies, I bought a pile of modern Kynoch brass from England, which is SALTY, but worth it, if I want this gun to shoot. Boy wouldn't it be a relief of every sort to take ye olde 450 NE and quickly re-form it to the 450/400..... Maybe I will find someone around here (central PA) who has a hydraulic press.
