Koji, Good information. Car was finished slightly after 9 E (glass date- all should be about same) 9=1969 E=May
Engine stamp confirm motor is 302 / 026 and motor parts cast by Ford on 8G5 1968 July 5th.
Check the block casting again there should be a date under it close to 8G5. Your block may have been replaced sometime in the past. (all Ford engine parts will have similar date codes, again most cannot be seen except intake and block), no need to measure as 289 rods have different part numbers than the 302 (see mustangtek.com), Your intake says your car was born with a 302.
Euro cars have a few differences - Odometer in kilometers , taillights in 2 colors, no side markers in rear.
You may have your original heads, the cost of purchase and shipping may not be worth the time and money unless yours are in really bad shape. A little porting and a light cam would be a better investment, head dates and castings cannot be seen unless parts are removed.
Other quick way to identify 289 vs 302.
If there is a number stamped on flat area on 302., that block was stamped for a Ford vehicle therefore not the original DeTomaso block.
