Doesn’t apply to your situation.
The adapter mounts are used when the earlier 2-bolt mount blocks (like your 428) are transplanted into later second-gen Firebirds.
455 blocks have the correct 5 bolts per side to accommodate mounting in big Pontiacs and the later Firebirds. Not sure exactly when the change was made but generally 1969 and down blocks had only 2 bolts for mounting on each side.
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1964 Tempest Coupe LS3/4L70E/3.42
1964 Le Mans Convertible 421 HO/TH350/2.56
2002 WS6 Convertible LS1/4L60E/3.23
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