basically,youll need the frame mounts from a six-equipped jeep.the v8 radiators are larger,but the outlets are on the correct sides,and ive never heard anyone complain about their radiator being too big.is your six coming from a wagoneer? if not youll need the throttle linkage and throttle pressure linkage from a six equipped jeep(since you have an 81 it has a 727,yes?or is it a manual?) and also the engine brackets and rubber motor mounts from a six equipped full size.the cj engine and frame mounts are differernt.they use the same rubber mount,but the brackets are differernt. youll need to do little stuff like reroute some wiring and hoses,but yes,to answer your original question,the 258 shares the same bellhousing as the v8s. i think some of the v8s even use the same flywheel as the 258.
i dont know that fuel economy is a good reason to swap in a six,nor is weigth savings(i think amc v8 weighs around 50 lbs more than six).a good reason is that it is a tough,durable engine. but im not sure how much less fuel youll use.it takes some foot in the throttle to get a big jeep moving with a six.and being abount as aerodynamic as barns, youll use alot of gas going 80 mph no matter what engine you have.mine uses alot of gas(not sure ho much,speedo has never worked,and it would be off anyway),but mine turns 38 inch tires with 3.31 gears. i use every bit as much gas as my friend with a 360,if not alittle more.in all fairness,my engine is tired,and im sure fuel economy would be much better if the tires didnt put it so below its optimal rpm powerband.
hope this helps.
85 grand wagoneer
258/904/twin stick dana 300/dana 44/amc 20
38x15.5 gumbo mudders
snorkel/dual batteries/onboard air/"custom" convertable
3 inch body lift/mostly stock suspension/"modified" fender openings
custom front/rear bumpers and brushgaurd