Arithmetic overflow error varchar to numeric
You get the error because essentially at the end you're dividing an 8 character long string by a numeric value which is has precision as 3 like this example below

DECLARE @num VARCHAR(8) = '12356456'
DECLARE @den DECIMAL(3,0) = 121

SELECT @num / @den


SQL Server internally tries to convert VARCHAR(8) to DECIMAL(3,0) and fails with your error. 

CAST([Column2] as numeric(8)) works because now VARCHAR(8) can be converted to numeric(8).

You don't really need to convert DATEDIFF(week, CONVERT(date,convert(varchar(8),Column1)), GETDATE()) to VARCHAR(8)