Yep, that is the bracket for the backdrive shaft that you show in the last pic. If you had to put the trans in 4th to remove the key, you will need to get a bracket that works with the Muncie trans. If you didn't need to put the Saginaw in gear to remove the key, then the shaft had already been over ridden and you won't need the bracket for the Muncie.
Also, the Muncie has a Super Shifter, which will require major floor butchering to use and if console equipt, the console will need to be removed. I suggest calling Hurst to see what Competition Plus shifter your year and model would take and look for one. The correct linkage kit would have the back drive bracket then too.
As a side note, the throwout bearing on the Muncie is backward, if you hadn't noticed. Don't try to install it that way.
1967 GTO; Gulf Turquoise on Black, 4 speed, buckets SOLD
1967 Camaro ss/rs 350
1966 Chevelle SS 396
1966 Chevelle Malibu
1970 Chevelle SS 396
1962 Impala SS
1952 Chev 2 ton flat bed
1938 Chev coupe