[Dominik]

Bob,

there seems to be nothing wrong with your approach. Hanging pedals with dropped footbox. Just widen the existing (?) cutout in the floor for the standing pedals and make up a pan to drop the floor for your heels.

the original pedal box, if you have it, will not accommodate two heels. It's not wide enough.

also, keep an eye on the throttle pedal. I mean the right foot. You may be too close to the main rails to fit a dropped box here...
Or angle your foot slightly.
Modify your foot! haha...

On the other hand, I don't really see the benefit. My 917 also has standing pedals on a level floor and I raced 24h Du Mans with it - I wish! ;-)

Dominik