Changing from solid to vented rotors shouldn't be that involved. I admit that I haven't done this conversion in 140-160 models, but I have done them on earlier 240's (e.g., an '80 with solids converted to vented), and if they're the same basic arrangement, this is what I found:

1) You take the rotor off the hub and slide on a different (vented) rotor. The "hat" of the rotor (which sits over the hub) is the same thickness as the original solid rotor -- the studs don't have to be any longer -- and only the "disk" part of the rotor is thicker. Remember that the studs are NOT going through the vented part of the rotor.

2) Yes, you also have to change from a solid's caliper to a vented's caliper, but again the caliper's pistons' displacement isn't different, so there is no difference in how much fluid needs to be displaced by the master cylinder to activate the brakes -- they don't need different master cylinder. The differences between a solid rotor's calipers and a vented rotor's caliper is only two things: (1) the mounting bracket (to position the caliper so that it's centered over the rotor's thicker disk, and (2) the space between the caliper's inboard and outboard sides (i.e., the distance between the pistons on each side), to allow more space for the thicker vented caliper to fit in-between. The vented brake's pistons do not move any farther than a solid brake's pistons (they are not farther from the surface of the disk -- consider that the pads are not any thicker), and a different master cylinder isn't called for.