Manual compared to Power. It depends on the set up. Some power setups will change the mechanical linkage ratios. But in your case, its not too useful bit of information.

Your decision should be based on:

• Whether the master cylinder will retain your pushrod.
• Whether the master cylinder has a large rear resevoir for the disks.(Some are just drum brake masters with the residual valve removed from the rear outlet).
• Total area of pistons that need to be displaced. Less or smaller pistons will need less volume from the master cylinder to move the same distance.

Cnxt's rules of thumb are good. A smaller master cylinder diameter will require less force to develop the same pressure. The downside is it will have to move further to displace the fluid needed to move the caliper's pistons.

Assuming your conversion is to a stock type caliper -
here's my work in progress crib sheet for the A-bodies.
Feel free to add, question or correct (especially if you have a documented source).

years___Application_____________p/n_______Bore
1966... Fixed Ft calipers, Manual................1"
1966... Fixed Ft calipers, Power..................1"
67-70.. Fixed Ft calipers...........2808600.....1"
71-72.. Fixed Ft calipers...........3461178.....1 1/32
73-74.. Floating caliper, Manual..3580146.....1 1/32
73-74.. Floating caliper, Power.. 3580112........15/16

1976?...............................____4647..........15/16