Finding The End Of Rows And Columns And Taking Union Of Sets

I have a code in software that exports results to excel. But it stores the numbers associated with indices as text. I need to change all of them to numbers. So I have written the following code, that search through the folder, converts text to numbers and colses the file.

Sub ConvertText2NumberFiles()
Dim v As Variant
Dim rng1 As Range, bk As Workbook
Dim i As Long
ChDrive "C"
ChDir "C: ...analysis"

[Code]....

The problem is it takes time becuase it is going through A LOT OF cells. I only need at most the first 4 columns and the first 2 rows. So I wanted to use the union. I wrote the following:

Set IndexColumns = bk.ActiveSheet.Range(Columns("A") & ":" & Columns("A").SpecialCells(xlLastCell))
Set IndexRows = bk.ActiveSheet.Range(Rows("1") & ":" & Rows("1").SpecialCells(xlLastCell))
Application.Union(Range("IndexColumns"), Range("IndexRows")).Select

Which doesn't work! Morover it is only for one column and one row.

NOTE1 : Like any matrix, the intersection of index columns and index rows is blank!
NOTE2 : The lenght of indices is different from one file to another