does the top of my header tank need to be higher than the radiator?

Thanks

If its part of the water system, it should be at the highest point in the system, so that could be higher than both rad and engine

Level in the tank just needs to be level with the top of the rad core or slightly higher.

An expansion tank lower than the rad can be made to work if you have a bleeder/filler at the highest but it is a pain in the neck for airlocks if you get a leak.