my daily driver is a 1990 ex cab 4x4 pick up truck. i has a 318 with an a518 automatic trans. [727 with overdrive]. my question is this, will a slant six mate to my automatic or will i need to look for another trans?

With a slant-to-smallblock-trans adapter plate yes. But I think this may be the least of things you need to consider.
I'm assuming the truck is a dodge ram fullsize?
If it's a dakota you're in for much, much more work.

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72 dart acquired 12/08 /6,holley2b, mopar perf intake, mopar perf header, cam, built motor,904 finally re-installed, 8.75....still needs sorting out.
99 dakota 318 5 speed reg cab picked up 9/09; Trans finally fixed! woohoo!

yup, fullsize. ive got the engine, mounts, and all of the ignition components that i think i will need. im a welder by trade so i plan on doin the exhaust myself. do they realy make an adapter for a v8 to 6?

Yep, used to be a factory part in some vans and pick-ups, taxis etc. with 727s behind a slant IIRC.
Sounds like you have it thought out pretty well, being able to weld is a plus!

Things that pop out to me when I think of this swap:
Wiring,
Fuel line,
Exhaust (which you have covered),
Mounts (which you have),
Adapter plate,
Radiator and fan.

Seems like you may need a specific/aftermarket flexplate, hopefully someone who has run an a518 behind a slant(there are a few here) will chime in.
Definitely use the search function and read the articles on the home page to learn more.
As for the adapter plate I would put a request up on the parts wanted forum, AFAIK noone reproduces them currently but they have been in the past.
Best of luck I think you can probably handle the rest.

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72 dart acquired 12/08 /6,holley2b, mopar perf intake, mopar perf header, cam, built motor,904 finally re-installed, 8.75....still needs sorting out.
99 dakota 318 5 speed reg cab picked up 9/09; Trans finally fixed! woohoo!

Also being a '90 you will be able to keep your current (mini) starter which will be awesome for you clearancewise if you decide to run Dutra dual manifolds or a header(s).

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72 dart acquired 12/08 /6,holley2b, mopar perf intake, mopar perf header, cam, built motor,904 finally re-installed, 8.75....still needs sorting out.
99 dakota 318 5 speed reg cab picked up 9/09; Trans finally fixed! woohoo!

I like the slant six and all, but honestly, it would not be my engine of choice for an extended cab 4x4 pickup. The only way I would use a slant in that truck is if you rarely used the four wheel drive, the four wheel drive was part time, and if you don't often haul heavy loads. Otherwise, you will get better performance and fuel economy with the 318.

Slants are great, but you have to apply them correctly.

If your truck was a 5 speed Id flyatter.

Im with Reed on this unless you live in the flattest of prairies and only drive downwind.

You will eat up that auto right away.

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Yeah....Im the one who destroyed this rare, vintage automobile.....