Door replacement - will door still be solid after cutting?
On numerous occasions I have had to cut hollow core doors off more than the available blocking in the top / bottom of the door. What I have done in these instances is to take the cut off bottom piece and then reclaimed the inner block for re-use. If a table saw is available it is quick work to set the fence just right so that a couple of rip direction passes cleanly cuts off the outer laminations. Alternatively the laminations can be removed using a plane. 

Once the block is nicely cleaned up it can be glued and slipped right up into the open bottom of the door. Use a couple of 1x4 boards on either side of the door and some good sized C-Clamps to tighten up the glue joint so that the glue can dry without any gaps. 

Note that often the inside of hollow core doors has an internal webbing of cardboard material to bridge the gap between the two lamination skins of the door. Sometimes it is necessary to use a chisel inside the cut open bottom of the door to scrape back the glue joint of this web material on the laminated panel. If this is not smoothed out the re-claimed bottom block will make the laminated sides bulge out when trying to glue the block in place.