Cambo, welcome to the forum. Your engine could very well be a 390, 406 or 428. It depends on the blocks cyl bore size and crank stroke. The 390 has a 4.05 bore and a 3.78 stroke. The 406 a 4.13 bore and 3.78 stroke. The 428 a 4.13 bore and 3.98 stroke. You can easily measure the crank stroke thru the plugs holes. But you'd need to remove a head to measure and confirm to cyl bore. If the engine runs good don't sweat it. If you ever need to rebuild it down the road you can measure and ID everything then. Your block is a 66 casting. 6G15 is the date code of 66/July/15.