Selling my Iron Duke from an 84 and a guy asked if it is a direct fit into an 89 s-10, I am pretty sure it is the same engine but can anyone confirm this? Not sure if anyone knows much about s-10s on here but I would appreciate any help. Thanks

And by fit I'm guessing he wants to know if it has same mounts, sensors, coolant inlets outlets etc so he can just throw it in without any modification.

No.
Is similar job as using a V6 in 84. IOW this swap is a big job to a very big job and have to rewire everything to the engine and ECM for 84 because 87 and up L4 are very different engines and use different ECM. Plus many other things will need to be mod'ed or replaced because 84 was made only for the 84-86 engine. Even using 85-86 L4 engine has changes and needs some work to fit 84.
And doesn't add much power for the effort and money spend.
More See my Cave, Duke Quick Ref
Then add truck is likely VIN A or E engine and many won't fit 87+ VIN R engine water pump parts and starter holes on wrong side at minimum.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

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Okay that sucks, don't want to sell somebody something they don't need. Thanks a lot for the info! (PS it looks like your site has a ton of good info but for some reason all the links on the left are overlapping the text and I can't read half of it.)