Series gearboxes evolved over time, but the only difference between the four and the six is the bell housing. If there were no changes to the bell housing in the time between the two boxes, the bell housing can be swapped. Even if this is not the case, it should not be too hard to find the correct bell housing. I seem to remember that there was a change in diameter of the layshaft that resulted in changes to the bell housing. 
But check the letter suffix on the serial numbers of the two gearboxes (just in front of the transfer case,on the RH side of the top cover below the square cover plate which is at the highest spot), if it is the same, no problems, if not, further investigation is indicated.
John
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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