How hard/expensive is it to switch out gearing on an A-body?

To swap from auto To manual you will need;

The pedals from a like car to swap in, the rod that goes from the pedal to the "Z" bar and all the hardware that attaches the "Z" bar to the inner fender and bell housing. There is the fender bolt, washer & nut to hold the Z bar and the plastic parts that go inside the Z bar on both ends, a pair of nut and bolts to hold a metal plate on the transmissions bell housing.
Said bell housing, fork, return spring, adjuster rod, nut and washers...

If I were to do it I'd forget about the clutch linkage, Z-bar, etc... and just get a clutch slave cylinder & hydraulic throw-out bearing. There are kits available. You still need everything else, including the pedals.