Originally Posted by Father Of Giants

All 90 - 91 350SDL's and 350SD's have the 3.5L.
The odds of it being swapped to a 3.0L is next to none, the factory only put in another 3.5L as far as I'm aware.

Also the oil filler cap is not original, how in the hell do you loose that? The original crankcase vent tube is plumbed into the top of the airbox when it could clearly just go into the hole coming out of the intake tube right beside the dipstick.

Whoever had this vehicle had NO CLUE what they where doing. I hope you're not going for the motor, it's probably shot.