I just use standard Craftsman and SnapOn combo wrenches. Some people remove the cover stud from the center to get more room.
You can also buy tappet adjusting wrenches that are made very thin. Or grind down what you have to fit. ERC
Some of the dollar stores carry cheap, stamped, sheet metal wrenches. It's not like they get used much or need to torque down hard. Just keep them separate and only use them on the pushrods.
Hitchcocks sell some 'spanners' that are made for the job.