Here is the correct backing plate for 66-70 B-body.
Power has a round hole and manual has a diamond shaped hole.
I might have one. Which did you need? Manual, right?
The ones shown from Dr Diff don't fit your car. Those are for 70-4 E and 71-4 B.

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72 Satellite Sebring Plus 440, 72 Dart 5.9 4-spd, 68 Valiant, 73 W200, 78 D100 sb, 78 D200, 98 DAKOTA, .
Moparmarks Parts & Restorations
Desert Mopar Metal
Grand Jct. CO
970-261-7039
http://moparmark.com/
motormangj@gmail.com