Shuie, two to three inches per side. There are some things to be careful of, though, and lots here has been written on the subject.
The bearings, leaves, grub bolts, steering box, brand of brakes, wheel width and fender type are all factors to consider.
Please be careful to measure all of your components as a composite measurement before you cut anything. Being off by even a little bit could have disasterous results.
I have the narrowed beam, dropped-spindle, disc-brake combo on a straight-bodied car, and there are limits to my car's handling as a result.
I'm absolutely not saying it can't be done, just cautioning you to measure twice and cut once.
I think I might have the most tinkered-with Speedster in North America. I'd hate to see anyone have to spend the same amount of time, energy and money I've had to, just to fix mistakes in geometry.
Any chance we'll see you at Carlisle?