Ahhm...my so-called unreliable sources tell me:
With the V8s the primary choice among Wagoneer buyers, the 258 cu in (4.2 L) six-cylinder engine was dropped in the 1970s, only to return as an option when Jeep sales – particularly of the high-volume Cherokee – were hit by the 1979 fuel crisis. (The Wagoneer continued to sell relatively well after production dropped to 10,481 in 1980, but increased to 13,741 in 1981, 18,709 in 1982, and 18,478 in 1983.) When reintroduced, the engine came with manual transmission as standard equipment, but in 1983 automatic transmission with “Selec-Trac” four-wheel drive became standard. With this combination the Wagoneer achieved EPA fuel-consumption estimates of 18 mpg<SUB><SMALL>-US</SMALL></SUB> (13 L/100 km; 22 mpg<SUB><SMALL>-imp</SMALL></SUB>) city and 25 mpg<SUB><SMALL>-US</SMALL></SUB> (9.4 L/100 km; 30 mpg<SUB><SMALL>-imp</SMALL></SUB>) highway – outstanding for a full-size SUV.<SUP class=reference id=cite_ref-auto.howstuffworks.com_4-0></SUP> This allowed the company to advertise good fuel mileage, although the more powerful 360 V8 remained popular with certain buyers despite its greater thirst for fuel.
Thats supposedly 7.9kms/litre in the city and over 10 on the highway.....though personal recollection of our Cherokee when it ran the good old 258 is between 6-7kms/litre at best.
I do believe that the Wag/Cherokee gear box is probably different from the CJ7 one which you have. Would be interesting if you could identify which one is it, possibly on of the following:
The T-18 : Jeep T-18 Transmission is a heavy duty four speed
The T-170s: Jeep Transmissions T-176, T-177, and T-178
All the best for the swap!