what size is the hole in the floor under the shock tower in the rear hatch on a 76 280

You mean the hole that give you access to the 24mm head of the long bolt that is the Moustache bar end mounting point?

Its about this big :

'------------------------------".

Or at least a 24 mm craftsman socket has about 1- 1.5 mm clearance around it when its on the head of that bolt. There's not supposed to be a plug to cover that hole.

Wow maybe that's why i can't find one thank's .

There's a picture here, http://atlanticz.ca/zclub/techtips/moustache/index.html, 5th or so down.