Generally, close is good enough. Range will decrease slightly if the
antenna is not ideal, but that is about all.

The ground plane is what you have and should be ok. The whips are used over
the entire range and there is little to be gained by cutting it for an exact
frequency. The ball on the top is ignored.

You do not need to be a rocket scientist to install an aircraft antenna.

The biggest issue is the connection of both ends of the coax. I have seen
some very poor work by otherwise excellent mechanics.

Colin