I don't have the Pentax FA 28-70 f/4.0 but if I were you, I would keep the Pentax and use the money for a good prime or zoom not overlapping the range that you have. That is of course, if you like the Pentax lens for the range and aperture that it performs for you. After trying out many lens, I have to say that a lot of f/3.5 and f/4.0 lens are just as good in daylight shooting.
The LBA cracks up on me really bad that I got both Tamron 28-75 f/2.8 and Pentax 28-105 f/3.2-4.5, along with a good SMC FA 28-80 f/3.5-4.5 power zoom. There is no doubt in my mind that the Tamron is a superb lens for image quality, build quality, and fast and sharp lens at wide open aperture. But having 2+ lens in similar range would make your life tougher when you have to decide which one to sell.
My life is miserable after having all good lens in that same range and I hesitate to sell one over the other. It is a good problem to have with too many lens but if I can do it all over again, I will always prioritize with less is better and go with prime for longer term and keep usable zooms around such as your Pentax.
After test shots with Pentax SMC FA 28-105, I decide to keep it longer along with the Tamron. One thing that I especially like about the Pentax 28-105 is on its small weight and size and in terms of traveling needs and walk around needs, I will pick the Pentax FA 28-105 any day over the Tamron. Tamron's fans, please don't flame me as I do love the Tamron but not in its weight. And you have to compare the weight of your current Pentax with that in the Tamron if weight is a factor in your consideration for highly used zoom.
Just a thought from someone having a hard time to decide which lens in repeated focal range to sell.