The most important thing with your combination is what was the timing ? Especially when using nitrous. 440-1 are not nitrous friendly , at least not at the 300 up level , so they may have taken more timing out than usual.
Lots of variables here as far as quench , cam specs , chamber shape , to give recommendations on tuning . At least as far as your nitrous goes .
On motor alone you can probably run a hotter plug .
As with most nitrous engines being
Conservative on timing will go a long ways , also don’t let it get too fat or too lean as both can hurt parts .

Yes you can certainly run the -9 plug for motor pulls . Yes it can be .025 -.030 gap . When I use nitrous I run the -10 plug at .025 and I use a lot more than 300hp