not done this mod meself, but knew a little about them.
Volvo portals comes in 2 gear ratio: fast and slow....so make sure you had the fast ratio ones.
Obviously, you'll be needing to cut away existing mounts and weld on new mounts suitable to fit land rover suspension/shocks/pan hard/drop link etc etc.
You'll be needing new props as well.
If memory serves me well, new extended brake hoses will also be needed.
Extended wheel arches will also be required as the portals are wider than standard rover axle.