Sending Unit not working

I just put a brand new sending unit in and turned the truck on and the gas gadge goes way past full what could be the problem?

Could be a bad gauge in instument cluster. I would think that a resistance check should be in your near future. I know this is'nt optimum but you need to remove sending unit and actuate the float by hand while checking resistance at wire feeding to gauge, Check for major resistance changes while you are doing this. If it is changing resistance then there is very little else it could be besides gauge.

i changed the cluster today and both the sending unit and the cluster are brand new and the reding still goes past full.

You need to get wiring diagram and check for open lead in wiring.