I had a real PIA trying to put a 71/72 head on my 68 bonnie. I could not get the 72 rocker boxes on over the inner head bolts (motor in frame) and didn't want to have to unbolt the motor any time I wanted to pull the rocker boxes. So I pulled the roll pins and went back to 68 boxes only to find the indents in the head under the head bolts were deep enough to have a gap between the early boxes and the head, this connection is imperative as the 68 head bolts clamp the box to the head as a unit to affect proper torque value and clamping ability. So after a bodge with thinning down washers and three bond I didn't even start it up, still sits in my shop looking pretty with a boogered head fitment. If you're lucky enough to find a 72 head with no indentations it would work with early boxes.
my old '70 had a later head and rocker boxes, just used the later two part fixings with
hex's on on the inside and short bolts for the covers. It was tight getting the covers on and off
but was ok. Fabricated a head steady to suit.
Dave Madigan does adjustable pushrod tubes if needed.