70coupe

how can i tell by the numbers on my engine block if it was the original one or not?

Soaring

If it is a 65 or 66 engine, you can't possibly tell if it was the original engine. If it is a 67 and up there will be a stamped number on the block that corresponds to your VIN. If they are the same, then the engine is the original engine that came with that car.

66GTKFB

Somewhere near the rear of the block, the last six digets of your VIN is supposed to be stamped. Not having owned a 69 Mustang since 1973, I've never looked. There is the casting number and casting date code located above the starter, which needs to be removed to see all that. Let some of the late classic guys tell you 'the where'.
Jim

70coupe

i have the numbers they are........VIN: 0T01F167662
Engine Number : C90E 6015-C
0E29

66GTKFB

Does the number you posted from the engine match the number on the left hand side of your windshield and registration?
Jim