Drew is correct. Adding to that would be that the center motor mount bolt hole in the block is exactly the same distance from the back of the block on both the 250 and the V8 so the tranny position does not move. On the 6 this is the top bolt hole and on the V8 it is the bottom bolt hole. I would attach the upper mounts to the 250 and then add the lower mounts to them with the through bolts and lower it all into the frame. The lower mounts should line up with the holes in the front crossmember when the engine is back against the bellhousing. Now you know which set of holes to use. It is possible they might be off forward or backward by a fraction of an inch and that can be adjusted by loosening the tranny mount on its crossmember so it can slide as needed. The driveshaft yoke will accept this slight movement without any problem.
If the lower mounts are way off relative to the crossmember holes then something is wrong. They might be for a different chassis.
