I grew up around the big Pontiac's as a teen and we had a few neighbors with the 421 engine. Love the 421 powered big Poncho's.
Too bad that to my knowledge there are no 421 engine emblems available out there for the 1965 and 1966 full size Pontiac's for our hobby. You can see them correctly rendered on the 2+2 kit.
For the 66 Bonneville, they will look like that but need to be mounted low on the rocker panel just a bit back from the front wheel.
For the 65 Grand Prix, if you wanted it to be a 421, the emblems would need to located in the same spot as the 2+2.
Hope this is a little bit of help.