Big Block starter - Just needs shims?? *Photos*

[Hank70SS]

Definitely check the starter for loose bolts, has it shifted.
As far as the bell housing, in theory it shouldn't have an effect. The starter and flex plate are attached to the engine. The starter should work even with the bell housing and trans removed, but who knows, worth checking.
Does the starter gear spin when the bendix throws it out? Just saying the bendix may be working but the starter motor is not spinning. You can short the starter terminals with a screwdriver and watch what happens. Unless you've already done that.