What are you towing that weighs 8000# but only has a tongue weight of 900#?
If you are going to replace the hitch (and you should if you are towing heavy like that and your particular truck is 15 years old), why not just jump to a Class V for the extra $50 and never have to worry about your weights?
If you are towing an 8000# travel trailer, you will run out of tongue weight capacity on the Class IV.
If your truck has always been a southern truck, maybe your current hitch isn’t rusted much. If you are towing those weights with a weight distribution hitch you do want to replace the OEM hitch.