A 1/8“ thick 1” dia. washer goes BETWEEN each head and the bottom straddle mount (1 on each end of the straddle mount).
Then the bolts (7/16“-20 X 7/8” bolts 5/8“ hex size) get put in with no washers on the bolts (between the bolt heads and straddle).
The head is machined where this washer sits. The washer raises the strap above any unmachined nubs around the washer seat.
1)
It keeps interference at bay. If you look at the different strap profiles, you see the 1000 mount is also relived for nub clearance.
The 900 strap is not relived and when you see heads that are broke at the tapped hole, you will see most are 900.
Busting the mount lugs is because the washers are put on top smashing the strap into the nubs and splitting the head.