I run the 680, and its been great so far. I've had it in both my old '72 Mach 1 with the 351C and my '95 GT. In the '72 Mach 1, I've run it dead a few times when the car wouldn't crank. After a jump start, the battery takes a charge like new, and cranks fine again.
In the GT I have yet to have it not start the car, though I was nervous about it at the track at first, since I cut the car off and crank it so many times.
'95 GT - 351C 4v, 4.56s, C4
'85 LX coupe - 5.0 stock shortblock, E7 heads, T-5, 3.73s, performer rpm intake & 600cfm eddy carb - [email protected]
1.684 60' NA