The best oven for roasting beefin an AGA is the hot, roasting oven. As long as the joint is a fair size (2 ribs or more) then the meat will sap a lot of the intense heat of the oven during the initial 30 minutes then be cooked more gently for the remaining time (I go for 12 mins/lb for medium rare beef). Sort of like cooking on an electric cooking where you start the meat on a high temperature then reduce for the remainder of the cooking time.
If you are determined to use the slow oven I'd brown the meat well first, then pop in for 8-12 hours (sounds a lot but the slow oven is only about 80-90 degrees) for meltingly tender meat.