One of our members had been asking be about making valve adjustment wrenches. The typical of using a normal combination wrench requires welding on an extension or some other method of doing that.
In the past I had bought some extra long combination wrenches (tappet wrenches have long handles but are usually only found in 9/16" which actually works fine) and made 8 sets of valve adjusting wrenches and they were long enough without welding on any extensions.
In the picture I taped on a crude ruler to a set of Heyco Valve Wrenches so you could get some idea of how long they are. The Heyco Wrenches are longer they the Hazet ones but I don't think it is more then one inch longer.
The span of my Hand is about 3-1/2". Looks like for me an 8" of length would be about the minimum length.
Note that on the 5th cylinder the long handle can sometimes hit the Fire Wall.
You might need to thin the wrench head:
Hazet Valve Wrench Head thickness = 0.242
My Valve Wrench Head thickness = 0.225
Heyco = 0.212
See post #21 if you want to look at pic of the set
I no longer make.
Valve Adjustment Wrench Set: Homemade, Cheap, and Simple