The 340 is a larger bore shorter stroke and a 360 is a 4.00 bore with a 3.58 stroke. Sorry off the top of my head I remember the 360 but not the 340.The big question is how much you want to spend and how fast you want to go? Mirada is a cool car but very heavy. If your going to use either motor and replacing the whole rotating assembly I would price out a stroker kit.I run a Scat stroker (408) forged kit and love it in my 73 360 block. My Aspen is all steel with factory interior and runs a high 10.