There are a couple different kinds of stud that Ford used - all press in. Straight ones where you can actually do a decent lifter preload adjustment, another with a stop where you just torque down. These early engines are easy to convert to screw in - pull the originals 3/8", run a 7/16-20 bottoming tap down, screw in the new studs, add roller rockers and away you go.
Later GT40/whatever version - these have pedestal mount rockers. You can either have the stud boss machined .230, the existing 5/16" threaded hole drilled out and threaded for either 3/8 or 7/16 studs - or do it the easy way with a Crane mod kit and Crane roller rockers. Also, the GT40 heads - much larger comb chambers, i.e. loss of compression ratio, round numbers 50cc for a 289, vs 60cc for a late GT40---
Now, the clank - see if one of the rocker studs has pulled out or at least one or more of the rockers has gone out of adjustment - replace the locking nut if so, but occasionally the stud strips as well. Then, it might be a broken valve spring.