We describe a Monte Carlo scheme for simulating polydisperse fluids within the grand canonical ensemble. Given some polydisperse attribute sigma, the state of the system is described by a density distribution rho(sigma) whose form is controlled by the imposed chemical potential distribution mu(sigma). We detail how histogram extrapolation techniques can be employed to tune mu(sigma) such as to traverse some particular desired path in the space of rho(sigma). The method is applied in simulations of size-disperse hard spheres with densities distributed according to Schulz and log-normal forms. In each case, the equation of state is obtained along the dilution line, i.e., the path along which the scale of rho(sigma) changes but not its shape. The results are compared with the moment-based expressions of Monsoori [J. Chem. Phys. 54, 1523 (1971)] and Salacuse and Stell [J. Chem. Phys. 77, 3714 (1982)]. It is found that for high degrees of polydispersity, both expressions fail to give a quantitatively accurate description of the equation of state when the overall volume fraction is large.