I would be leary of any universal type of all thread. I have a genuine spring spreader used in Ford dealers in the thirties and forties. The threaded portion uses acme thread and a corresponding nut with a slight raised face on it to ride on the tube. I have not seen that style of threaded rod for sale. There is a reason Ford chose the acme thread.
Well, ok, maybe it is a wise statement to say don't use all thread, but if it is stout enough and the spreader is of sound design, it is safe enough. The one I made used good thick wall tubing in the centre and two pieces of all thread on each end. The all thread is 20mm diameter and is as long as it can be to get the most support from the tube, but with the least amount sticking out that would still work. As it was for a 40 type spring, it gets a straight shot.
I would not use a design with a bend in it, or with the all thread in the middle. That would be asking for trouble.
I would partially agree with Mikes statement, using all thread could fail if the design is poor, but if the design is sound, it can work successfully and safely.