The 4.3 Mercruiser has two bolts that go into the engine block. One starter has a staggered bolt pattern, the other one in line. Don't know about a support bracket. I believe the torque specs are 50Ft. Lbs.
JamesMc
Ayuh,.... If yer starter has the vacant stud on it's leadin' edge, ya just use a small piece of angle iron to go from there to the vacant hole on the block, just ahead, 'n above it, at the oil pan rail,...