You referring to this part:
There are two versions: With cast iron master cylinder and with aluminum master cylinder - which (I think) is a mid-year '78 change.
The top and bottom holes are the same. The middle two holes are in different location. The pictures above use the aluminum master cylinder.
I am undecided on using the above pictured part - so not willing to sell it (at this time).
It is made from thick steel. I would be willing to take measurements for you and I would recommend using a piece of thick aluminum plate to make one - which would end up being lighter (and can be painted to match body color . . . or black).
BudW