Ideas on what's causing my click/tick/rattle noise?

No he wouldn't have had to swap the head/ just the rockers and pushrods. The hydraulic lifter head doesn't care,
I forget what it is about the back bolt on the rocker shaft but something is different about that from a hydraulic lifter engine to a solid lifter engine. One has a regular bolt and the other has a shoulder bolt.
That said I cracked a head on my dad's 82 Gran fury /6 years ago. I put what I had laying around on that motor to get it rolling, which happens to have been a 63 head complete with drool tubes. I ran the original Hydraulic setup and I remember I had to drill out the bottom of the hydraulic lifter shaft, the top side was ok as was, drilled out the hole on the bottom of the shaft to match the diameter of the hole in the top of the rocker shaft and then all was good.