An 8L dated engine is actually for a 69 model year car, but I'm sure you already know that.. It makes a difference cause an engine dated properly for a 681/2 CJ or KR Shelby is worth significantly more than one for a 69 model year car.
That said, a lot of it depends on how complete the engine is, what was done on the engine, and who did the work. Standard bore is good, but a lot of people don't trust "rebuilt" unless it's a known shop. Do you have some Pic's, or a list of what was done to the engine ? Also what accessories are included can make a huge difference in price if they are correct and properly dated. (carb, fuel pump, distributer, bell housing, smog, ect)
I would say a long block, rebuilt properly with good parts would be worth $5K, then add up what all the included xtras are worth to you...
Now if the VIN stamped in the block matches your car...... That's a whole different story
Hope this helps, John