I am going to assume you are using the stock accy drive, yes you will need a tensioner for the belt to stay on. Be advised that 5.0 H.O. uses a reverse rotation water pump so that if you used a early cars atlternator and power steering bracket the water pump will turn in the wrong direction which is BAD! If I were you I'd find a 87 to 93 Lincoln, or Mustang and grab the complete accy drive off of the parts car including the power steering pump. The pump will hook up to the power steering your using with a parts store made pressure line the return line being a piece of hose just leave the air pump off and run a shorter belt. If you want to run a standard rotation water pump you will have to change the timing cover, water pump, and belt once you do this the pulley's will not line up properly I'd run the full serp setup out of a late model or buy one of the March Pulley kits or you could go with a stock V setup off a older car/truck 84 -86 -BKB

Sources: My 65 Comet B/FX clone