Biggest concern would be that the blocks are designed with the original mounts in those locations. Added addition stress to other areas not designed for it (due to vibrations and weight of the block on the mounts) I would advise not to do this. If this were some super cheap rat rod that you were throwing together as a cheap toy that isn't expected to last long I would think it would be ok. Also the stresses of heating the cast iron block and then welding too it would cause more problems and I can see maybe if it were a fully filled block with no coolant passages you may be ok.Fixing/reusing stock mounting locations is really only correct and safe way to do it.