What is an max acceptable amount of fore and aft movement ( wobble) for the flexplate when the engine is running. Zero runout is obviously the best. I put two flexplates in and both have about .040 to .050 I ran the engine last year and didn't have any problems. The rev limiter has a 7k chip. I am wondering if this is common with factory plates. And if so is there an rpm where they start to cause problems with harmonics and eventually come apart

You need to look at a couple of things. One, like obnoxious stated, get a quality plate.

Second, the run out you are seeing does not sound unreasonable, however, are you certain it is seating properly?  Do you have a PTO designed specifically for a flex plate?

Also I will add,it is important to check the crankshaft thrust run out. The harmonics will not come from a flex plate gyration, but it can weaken the flex plate center causing a very unfortunate failure.

GT