Here's my opinion for it's worth.
It all depends on what your goals are for the car. If you're building something other than a thoroughbred level car, then re-upholster if you like however remove the original seat covers with caution and either store them away or sell them. A good original set of upholstery is hard to come by these days. As far as I know, there only two major brands of aftermarket upholstery. TMI and Distinctive Interiors and both have pros and cons. I personally prefer TMI.
If on the other hand, you are building a thoroughbred level vehicle, keep the original upholstery. The aftermarket upholstery is not an exact match in physical appearance. You could replace the seat buns only however they are a bit too thick IMO and need to be shaved down a bit for a proper factory appearance.
Whatever you chose, best of luck to you!!