If your 91 model year short block is an LA 318, which it should be since it's a 91 and had a fuel pump block off plate, then your old oil pan arrangement from the 87 LA 318 that's already in there and proven to fit will swap right over and fit perfectly. The Magnum engines have additional mounting bosses for the 1994 and later truck engine mounts; there will be three bosses in a triangle pattern on each side of the block drilled and tapped for brackets, and they are just rearward from the LA style mounting ears. If you don't have those bosses, you have an LA block and nothing to worry about as far as oil pan swap from the old engine. I am also east of Lansing, basically in Farmington Hills area.
