yeah if by a 5.3 you mean an old school 327 and by a 5.7 you mean an old school 350, then it would work, but if you mean the 5.7 old school 350 and the newer 5.3 whatever cubic inches it is, then theres no way in hell. maybe you have the 5.3 confused with a 5.0 old school 305?
i doubt that putting 305 heads on a 350 would really make any difference. In fact i bet it would hurt your performance since i bet the valve sizes are smaller for the 305 heads or something, i have always heard that the stock 305 heads were worthless