There are lots of opinions on this, but generally, the engines that were installed in front of Powerglide transmissions had hydraulic lifters before they came into general use. By the late 50's, all the 235's had hydraulics. A quick way to tell is to pull the valve cover, and find a valve that's in the closed position. Clamp a vise grip onto the adjusting nut and rocker assembly, and hold steady pressure back against the pushrod for several minutes. A hydraulic lifter will bleed down, and the gap between the valve stem and the rocker arm will increase to almost 1/8". Don't worry, as soon as the engine starts, the lifter will refill and the clattering will stop.
Jerry
