Most 904's are compatible unless they are early, rear pump units, or you get to the lock-up versions then the torque converter gets different, you can tell by this, this is the shaft you mount the torque converter to:
Since this is a slant application, it would help with gas mileage to have the LU, so sure, have at it. Make sure it's in good health and you wire it up correctly, where applicable. Do drop the pan and inspect the bottom, any excessive black clutch material and metal flakes it's time for a rebuild.
You might want to think about putting a tf-2 shift kit in there, as well as a drain plug in the bottom of the pan while your there to to firm things up a bit as well as make maintenance easier if everything looks good.
If it doesn't:
I believe in 1980 they put in the desirable 2.74/1.54 1st and second gear sets that got big vehicles off the line and is usually equal to 1 to 1.5x differential gear change up. You can swap them in if you wish, if you end up rebuilding it, you can get all the pieces for wittrrans.com wonderful people to deal with.
Here is the good rebuild kit if you need it:
http://www.northernautoparts.com/ProductModelDetail.cfm?ProductModelId=9099
If you search the site there are ton's of instructional material on how to get it done. I've done one in my kitchen, so that "Oh, I don't have the space." thing is hog wash.
A904's are simple to deal with and a waste to let some one bend you over on a rebuild for one. Check 3 times install once.
