need to replace the idler arm on my 1965 malibu,dont own a torque wrench is there some other way to tighten the castle nut without a torque wrench so as not to overtighten. thanks john.

There is no "true" torque for that nut. The spec is like 75 ft lbs then tighten until the next set of slots line-up. Not a real accurate torque, kind of like torque to yield bolts. You are basically just making sure the minimum torque is set. I haven't used a torque wrench on front end parts in over a decade. Just make sure it's tight.