More or less the same. Maybe a few pounds saved.
The stock K-member mounting pads, steel engine mounts, insulators along with the hardware bolts, washers and nuts will come out to be a little more in weight than a whole cross section aluminum plate, plate mounting pads welded onto the frame, engine/transmission limiters and all of that mounting hardware bolts, washers and nuts. Plus remember that you probably have to weld shut some spaces where the mounts were attached to the steering box mount on some cars.
You'll also have to shave the same difference off the water pump housing on the engine mounting surfaces to make up for the plates thickness.