What you might be looking for is a contant-velocity drive, these are used on some snowmobiles and ATCs in place of a conventional clutch and gearbox.

Essentially the pulley is built with two opposed cones tapering inwards forming a "V" between them. A wide and thick "V" belt sits between the two cones. As the speed changes the cones move together or apart with the belt seeking a different ride height in the "V" groove. The different ride height generates the variable drive ratio, 4:1 is quite feasible. A simple flying weight governor (similar to your advance plate in the distributor) drives the cones in and out to regulate the speed.

My truck has two 135A alternators standard, probably as Fords answer to this problem. Two alternators driven off the same belt with identical pulley diameters should share the current fairly evenly. Since the current per alternator has been halved their output voltage should increase - hopefully up to the desired voltage. This is a more practical solution and provides redundancy as well.

Cheers, Mark