Hi m1972
I'm from Namibia and can help you a small bit... Maybe just point you in the right direction.
Huntafrica Namibia Safaris - James Chapman & Koos Pienaar are very good choices and they average very good on their ivory every year. But you'll have to book way in advance!
SMJ Safaris - Stephan Jacobs has the Nyae-Nyae concession area which is world-renowned for its big elephant. The previous concessionaire (Kai-Uwe Denker) shot three 100 pounders in the ten years he hunted the area and had an average of over 75 pounds for some seasons. But honestly, I don't know how Stephan is doing with concession currently. However, I can assure you, Stephan has the best area, but its definitely the toughest area to hunt too. Like previously mention, Karl Stumpfe from Ndumo Hunting Safaris would also be a good call. I don't know what poundage he averages per year, but what I can tell you is he is a good PH and is a great hunter.
As for Zimbabwe, the PH that shot the 100 pounder last year near Hangwe National Park is Russ Broom if I recall correctly... I'm not 100% sure!
Hope that info helps a little!
Regards, Jacques Strauss