Framing around depressed area of floor: a good plan?
the 3/16 x 1/2 overlap tab is a little thin in oak, but it will probably be fine given the other structure you propose.

i will offer this though: you are mental!!!

it seems like your plan is well thought out, but wouldn't it be much easier to just make a hole in the ceiling?  then its just plaster and paint?  if you ever decide to not have the clock or you move out, its far easier to patch a ceiling than a floor. 

if it was me, i would just cut the clock.  to remove 8" or so out of a grandfather clock may be impossible (style of cabinet, length of pendulum, etc).  however, many times, the casework of the clock is just a hollow carcass with what is essentially a baseboard and legs bolted to the bottom.  if you can do what you propose, surely you can build a modified bottom for the clock? 

or sell the clock and buy a shorter one??? 

however, its your clock and house and i am sure you have your reasons.