I have set up a Simplex TdF and made it work fairly well (which is possibly as well as they ever work!), but I certainly would not claim to be an expert.
I'm not clear about exactly what your problem is - are you saying that if you have the chain tension correct it won't change, or that you can't get the tension right?
I suppose you have got the jockey cage the right way up - it's not rare to see them fitted upside down so that the the top jockey wheel is too far away from the sprockets, which might cause the sort of problems you describe.
It's worth mentioning that these old 'plunger' mechs were never easy to set up, especially in comparison with parallelogram gears, and I'm talking here about a time when they were current and new spares were readily available.