Hydronic heat Loop system...???

Luigi

Good Afternoon...I need to replace one of my old cast iron base board with a new style base board maybe Slantfin or Runtal steel radiator...the system that I have is of a single loop Layout (parallel) that feed each radiators independently and cool side of the radiators are connect to the return of the same loop. The concerns that I have are listed below..
·the old type had 1/2 copper and most new ones have 3/4" tubing
·no diverter tee's were use so how those the water flow correctly to each radiators
if the least path of resistance would be the main loop
·should diverter tee's be used when installing new baseboard
·what would be the best approach…
thank you…Luigi