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.

Possible driveshaft. Shifter boot and trI'm ring to hold shifter boot down. Shifter and rods and shifter handle. Clutch alignment tool
For the pressure plate and clutch disc. Possible Offset dowels To align the bell housing up. Trans mount. (At tail)

BUt before you do anything, the crank must be drilled in order to Accept the transmissions input shaft into the crank with a bearing.

If the crank is not drilled, guess what needs machine work first.