what is the ideal prop shaft height for 29' v-bottom twin small blocks ,,, boat weights about 6400 dry

Your boat has a nice attitude. What props are you using and how far are your prop shafts below the bottom? Did the boat come from Wellcraft with Mirage or Bravo props?

using 25p mirage splined and 23 mirage plus ,,,, center line of shaft is 7 1/4' below hull

The Ideal height will Vary by boat, Sometimes even boats from the same mold depending on what it is going to be used for. Racing/Cruising etc. If what you're going for is top speed then higher is better "To a Certain Extent" once you are high enough to lose leverage, Then you lose Trim-ability. You can get some back using props with more blade area, After that you need to start going back down

I wont do anything quick fastdonzi, where would start?

If you can try some shorty's that would be good, Actually you'd rather get high (Prop shaft hight ) by using shorty's. A Re-rig to raise standard length drives moves everything up. Can be done, I don't recommend that route for testing. A Re-rig of that type would cost as much as shorty's.