First off, it's apparent that it has a rear main seal leak ....
I think Toyota installed the steel adapter plate for a reason, prevents a leaky rear main from affecting the clutch components.
Why is it "apparent"?
The plate may slow it down, but will not prevent oil from the rear main seal from getting on the clutch disc if the leak is bad.
At the bottom of the bellhousing there is little 3/8" slot. Is oil visible and seeping out of that hole?
And secondly, an interesting issue with a leaky rad cap.
This should be an easy fix. A TVS Stant cap should be more than adequate. I would not spend $20 on a radiator cap.... but I'm in "frugal mode".