Those slant sixes were really great running motors. Friend of mine had a dart back in the 70's with one and I was surprised how quick the car was. We used to drag race our parents cars against each other, I drove a tr3, and friends with the dart and a 74 blazer. They were all 3 very close in speed. My dad also had a van like yours, I did a quicky rebuild with a ridge reamer, hone, rings and bearings.