Motor Mount Install Question

Your pass side mount is a truck biscuit mount. They are the same on both sides for those year trucks.

1979 DODGE D100 PICKUP 5.9L 360cid V8 Motor Mount | RockAuto

The mount brackets you've got look like they are to mount a small block to a slant six k-member. If the k-member you've got will accept slant six mounts, you can probably make what you have work. My Schumaker /6 to SB mounts use a thick pass side mount too. I'd bet it's about the same thickness as the one you've got. You just need to figure out what your k-member is made for to know if what you have will work. If it's for a slant six, cut the tabs off that mount and bolt it up. Verify what you have before cutting anything though.