The beam fits perfect, you will as you say if using sapphy arms need to use the sapphy arb, which is miles better tbh, unless you just use your 3 door arms sapphy beam ?
The best way to mod it is to get a few bits of say 4-5mm thick metal 1 1/2" wide and about 5" long, put the arb clamp on this, and mark the hole position and drill/tap the hole, then easiest way to sort other end is weld another "eye" type fitting as the other end, make this then drill tap again, offer up to the car to get good placement, then mark it, then measure to ensure centrally located, mark the position of the bolts that need to protrude through the box section and drill out these holes larger than the bolts.
Weld plates in position job done.
tabetha