The problem of optimizing a cluster of isothermal or isoflux parallel-plate channels where the coolant is a metallic liquid is addressed. The pressure difference is fixed, and laminar forced convection is caused by the simultaneous development of velocity and temperature from free-stream conditions of the liquid. The Fanning friction factor is invariant with the fluid. However, local and streamwise-mean Nusselt number distributions for each heating condition are carefully computed exploiting the physical analogy between transient conduction in a flat plate and steady temperature development inside a parallel-plate channel under the premise of slug flow. The qualitative influence of diminute Prandtl number liquids (Pr = 0.01 and 0.005) is reported in terms of the optimal heat transfer and the optimal plate-to-plate spacing for the two heating conditions employed.