It sounds like you've made a mistake; one that I've made myself on my 66 Mustang. Without oil pressure, hydraulic lifters won't just spring back to their fully uncompressed state immediately. After rotating the crank to make sure the valve is closed all the way, allow the engine to sit in that position for a couple hours to give the lifter time to 'relax.'
Hydraulic-lifter pistons have a very limited amount of travel or preload: .020 to .060 inch. On top, that means approximately 1/4 to 3/4 turn at the rocker arm. When adjusting valve lash, you want the lifters smack on the cam lobes’ heels (valves closed) on compression stroke. But honestly, cam manufacturers offer even more precise advice: Slowly turn the crank until each valve closes completely, then, make your adjustment.
Turn the pushrod with your fingertips while tightening the rocker arm adjustment nut. When the pushrod becomes ever so slightly resistant to your fingertips, turn the adjustment nut 1/4 to 1/2 turn. Though Ford suggests 3/4 turn with some applications, this is too much. If you’re running poly locks, tighten the Allen screw lock. Do this in the engine’s firing order, one cylinder at a time.
You don’t know if you are successful until you fire the engine and it is at operating temperature. If there’s significant rocker arm noise, there’s too much lash and you need to go a little tighter. Some aftermarket rocker arms, such as the Comp Cams Pro Magnum or Ultra Pro Magnum, make a soft clicking sound, which makes a Cleveland sound more like it has mechanical tappets, but is of no consequence.
With roller or flat mechanical tappets, valve-lash adjustment is simple. As with hydraulic tappets, follow the firing order with both valves just closed. Valve lash between rocker arm tip and valvestem is .022-inch cold for both intake and exhaust. When you start the engine, you should hear uniform rocker arm chatter. Any loud clicking is excessive valve lash.