I not sureprising a 421 was not on there so I used its sister 389 (basically .030 smaller in bore, pontiac just over bored the 389 to get a 421 it was a cost saving thing they used the same casting.)..
No, not just .030" overbore but also .250" more stroke...
Pontiac
Engine.....Bore.........Stroke
389........4.0625".........3.75
400........4.12.............3.75
421........4.09375........4.00
428........4.12.............4.00
455........4.15.............4.21
Pontiac's can make some serious power...parts are scarce and expensive.
LO