You remove the Ashtabula 1-piece crank/chainrings/bearings/cups and buy an adapter that lets you use 3-piece alloy cranksets and 68mm bottom brackets with English treading.

Here's a commonly used adapter:

TruVative "American to Euro BB shell adapter"

I'm doing the conversion now on a '73 Schwinn Super Sport, and am installing a Campy Comp Triple 10-speed group on it.