Post the engine number - we can tell you if it is within the range that would be expected in 10/70.

L24 120799

Your original engine serial number should be something close to L24-016xxx - or it could be as early as L24-014xxx.   L24-120799 would have been found in Z's built around 08/72. Casting numbers on the Aluminum Head from 08/72 would be E88 - where Casting numbers on the original engine would have been E31. The Casting numbers are near the front (radiator side) of the head.  

The head has E88 on it 

 

That is unfortunate that the tag is gone. It does sound like the motor has been changed already No easy way to confirm without the tag. Still a good car nonetheless. 

It's up to you to do what you want to do with it. Sounded like you wanted more power. Turbo, Stroker motor, even up to a V8. Lots of options.

HybridZ is another good forum for swapped motors and more as far as mods, although we would still like to hear about your plans. 

Is this an original blue car? What color interior?