I ordered a new rod from the machine shop that I bought my engine rebuilt kit from and it is a different shape, seems to be more heavy duty then the ones that came in the kit, will it be alright to use, or will it throw off the balance of the engine. Thanks for any help... it is a 396 bb,

If it is enough bigger that you notice the difference it will not help. I have no idea how many different rods that could have been originally in a 396 but there are quite a number.