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[1969_Mach1]

So, if you have the first scenario listed above, I'd lean toward the 3.50:1 ratio.  My older brother had a setup like that back in the 1980's in a 69 Mach 1.  It was still fun around town and you can still do a little highway driving.

yeah, pretty sure it is 3.50:1 ratio its all factory stock minus the few things i changed. it is a Automatic transmission also.
my old 78 cobra 2 i had when i was 16 we put a shift kit in it and did some minor engine work and it would hit 2nd pretty hard and spin the tires.
got that one when i was 15 and parted ways after buying the 69 Mach 1, still miss that 78 had a lot of fun in that car.

If it's all factory stock it probably has a 3.00:1 rear axle ratio from the factory.  That was the most common with a 351W 4 barrel and auto trans.  If that's the case, the 3.50:1 gear ratio will make it feel like a different car.  Double check the rear axle gear ratio.  With an open differential (non-posi of any type), with the rear axle supported on jack stands and the trans in neutral, hold one rear wheel so it cannot rotate, rotate the drive shaft and count how many revolutions the drive makes to complete two revolutions of the free rotating wheel.  I.E., for an open differential with a 3.00:1 gear ratio the driveshaft will rotate three revolutions for the free rotating wheel to revolve twice.