need to know what type of engine
 

hi i have a 1966 mustang and i am about to rebuild the engine and i think it is a 302, but just to sure i got the block number but i have no clue how to read it the number is 3E25, what is it a 302 or 289

That number looks like a production date number. Like May 25, 1973, the casting number is near the starter and should start with a letter. Like "C", "D", etc.

Early 66 had only 5 bellhousing bolts. All 302's have 6. Also, I know this is the heads, but, 289 is cast in the heads under the valve covers and the heads should have a straight shaft for the rockers, not shouldered.