in addition to my 82 /6 d-150 i have an 84 d-100 w/a 318.the auto tranny needs replacing.a local junkyard has an early 90's d-150 318 w/ an auto overdrive tranny.
would this be an easy swap?

Do a search for Dodge automatic overdrive or A518 swap on Google and you will get your answer.

It isn't a difficult swap, but you should be aware ofa few snags such as getting the non-computer controlled tranny and wiring the overdrive switch.

Not clear to me...is the auto gone bad on the slant 6 or on the 318? If putting the 90 auto off a 318 back onto the 318, pretty easy. If putting auto from 318 onto a slant six block, adapter needed.

rock
'6d100

sorry about not being clear,the 84 d-100 has a 318 auto.the torque converter/tranny is dying(makes a dog-chain rattling sound).it shifts at the proper points but over the last year or so the gas mileage has slowly dropped from 14-15 mpg to 6-8 mpg.
i've had the local mopar guy take off the manifold and clean out the carbon build up.after this it would burn the tires,lots of power but still 8 mpg.the engine runs good ,i've has had 2 different carbs on it.1 holley ,1 carter.no difference in the mpg.
so the tranny is all that is left.and the local junkyard has an early 90's overdrive auto in a truck that was hit on the passenger side.i'm hoping that it would be a fairly easy swap that would result in a little better mileage than i was originally getting.

Oh, ok, its easy.

Reed's right about grabbing the other stuff, and I would get lots of other junk such as mounts, driveshaft(for u-joint flanges) wires as far as possible and so on. As to price you can get a rebuilt one with no goodies to run it for about $700 most places although many also get a lot more. If you can get a known good one it may be worth your while to get it and just buy the controlling goodies, for instance. The main reason I didn't make this swap is I lucked on a 727 at a great price. I have a 3.91 rear end ...what are you running?

rock
'64d100