Gentlemen, Start you engines

the 904 in your car now will not work behind the small block V8 so you will have to change the trans

Yes you can get a 9" out of a truck cheap enough, but then you will need to get it shortened, new axles, weld on the perches, U bolts, plates and proabably a custom made driveshaft. I think at the $400 offered for the A body 8 3/4 is the better deal. Bolt it in get a driveshaft for the dart with 8 3/4 ( I think I had to have a 53" Dshaft) and you will be driving cheaper than the 9" Ford

Dream Big, collect the parts as you can, and hope you get the car on the road in the future