Any particular reason you want to use the same head L/R (ported, better valves/springs, etc)?

If it came down to it, I'd be tempted to keep any parts off (if they are better than stock) of one of the heads and sell it and put those funds into the correct correct head, or sell the whole head and buy the correct head for the side instead of investing all the time and effort (and getting it machined, threaded correctly, etc) into finding out if the other head could possibly be converted to use on the other side.