agree with aggie72
Try it before you do any mods. I'm using one for VHF and UHF air with pretty good results.
Been through the same process about a year ago. The radio shack ground plane does work pretty well for aircraft band, but the elements, as stated above, are a little short. I used it quite a while though, then dropped it off the roof and broke one of the lower radials off, so I decided to use the plans available for a homebuilt-model, with the closer dimensions to that aircraft range of freqs.
It blew me away, improvement big-time over the radio shack stock model. And it still picks up the VHF range that I listen to, 155mhz, and does that very well also.
Try one, you won't be sorry! Then build yourself a copper J-pole cut for aircraft band too, another excellent project.