1UZ-HZ,
If you can, keep the flexplate since it's already done for you. Most converter shops can relocate the Ford converter legs to fit the Lexus flexplate. You might have to get a lexus converter to make sure the leg height is right. The button on the crank side of the turbine housing would have to be cut down to fit the Lexus crank. This button fits in the crank hub (on many cars) and centers the converter on the crank. If it's too small you might have to have a bushing made to fit in the hub to accept the converter button.
I haven't gotten to this point yet, so I'm really speculating at this point.