Stretch array in dynamic block - problem with default column numbers

I understand the new array system uses a 4x3 column row default. Is there a way to over ride this. I ask because it has reared its ugly head in a dynamic block i'm trying to build.

this is how it is built (i have shown what has been picked for which action)

The array has been set as 750 column spacing

this is what i am expecting to see when i stretch the block

but this is what i get

because autocad is immediately putting the default 4 columns in...I just can't figure out how to over ride it.

here is the dwg if anyone might have time to help out.

rollerrack.dwg

Dear All,

I've sorted it. Was thinking back to front...have atteched my stretch action to the array linear stretch rather than attaching the array to the stretch parameter...