When you pull on the downhaul, the only way the mast can go is to bend lengthwise. It cannot bend sideways as 1: you have spreaders preventing it and 2:the bend is going to develop along the axis you pull as the line you pull is captive inside the mast track.

You won't get the range of tuning you get on a newer mast, you can't induce prebend so inducing the prebend has to be done with the downhaul, helped witht he main sheet to start it maybe. But you'll get enough to depower and let the square invert. However I would not put a 16:1 on that old mast: something is going to break when you crank a 16:1.