Just done some experimentation on my being rebuilt1970 which I know has a straight frame and engine mounts. With the rockerboxes on tight, you can't get the timing side central head bolts in or out, particularly the inlet one. There is enough room for the primary chain side ones.
What I have seen done before now is file down the lower head bolt head on one flat to give the necessary clearance. Some preferred just to grind down a corner. A tube socket will seat on the remaining corners, particularly if you get the six sided sockets. Rough, but it works. The alternative is loosen the rockerboxes which allows them to be wiggled out.
Bruce - I am not sure they designed the heads to be removed easily- they were put in the frame fully assembled. I note that the Thomas Gunn overhaul manual
specifically states putting the two 3/8" head bolts in first and tightening before putting in any of the others. On disassembly the words are "these bolts will clear the frame if both bolts are lifted and the rockerbox tilted.