Dave, you beat me to it re illustrations, but I'll post anyway so folks can see them side by side.
A piece from the people who overhaul Allisons.
Yancey Allisons
To make a left hand engine from a right hand engine, you have to reverse the crankshaft, replace a regular gear with an "H" gear (this is a gear that skips over a gear that was used), and add a standard gear to reverse the prop rotation after you skip with the "H" gear. Every Allison engine has the ability to be reversed if you have an "H" gear and the added gear.
The right hand bank as viewed from the distributor end (rear of the engine) must have several spark plug leads interchanged becasue the cam lobes are backwards. It works out the same for the left-hand bank, somehow ... no left bank changes to the firing order
Basically, to make a left from a right, the engine must be almost disassembled becasue you must be able to get to the gearcase in front and must split the case and disconnect all the rods to reverse the crankshaft.
However, if you are BUILDING a left or right from parts, the difference in buildup is trivial, assuming you have an "H" gear, the new gear, and you know the plugs to interchange (this means rewiring the ignition harness on one side ... so it is MUCH easier to simply build a left or right wiring harness than it is to change one that is already wired).
The "H" gear:
A Standard gear is just a gear with a keyed center. An "H" gear looks like two standard gears joined by a small cylinder in the middle to skip over the gear that was formerly meshed by the standard gear. The new gear you add is to turn the skipped gear in the other direction. All gearcases have the ability to turn either way ... the gear bosses are in all of them, internally.
For both left and right engines, the cams turn the same direction and the crankshaft is reversed and turns backwards.
There is no other engine I know of from WWII that as so easy to make turn in either direction when being built up ... two gears, turn the crankshaft around, and change the right bank firing order ... that's it.
You need a starter that turns the other way, and you need an idler type gear to reverse the direction of the cam towers.
left hand rotating
right hand rotating
