I have a 350 300hp for a 69 camaro SS with an automatic. (HB block)
was rebuilt, has maybe 10000 miles on it. Whats it worth?

$1763.17.
How's that? Seriously, it all depends. No way to tell without a mechanical health-check and a review of the work quality performed on the engine.

Post the numbers (pictures would be better) of the pad stampings and the VIN stamping. To the owner of the original car, that engine is worth quite a bit.