I thought I would pass along a little information about welding 4000 series steel.  I am not sure what material the Lancair engine mounts are made from but if it is a 4000 series steel then the material must be preheated before welding and then post heated for 1/2 hour after welding to prevent cracking.  4340 steel is much more prone to cracking than 4130 but they both will crack if not processed correctly.  If I was going to repair a cracked engine mount, I would have it magna-fluxed after weld repair to check for cracks.  Very small cracks may show up, but if they can be removed with a little de-burr, then the welds are OK.  That is, they are OK, if processed correctly.
Craig Berland