Bob Mannel had access to Ford engineering files while working on his book. He gave me a rather lengthy explanation of why the six bolt design was adopted and it was clutch size related. The whole discussion included legal and cost aspects of shipping ready to use anything across the border, along with what Ford had capability of making or having made in Mexico at the time. The short story, going to a six bolt block was the lowest cost path forward. The conversion to the six bolt process started about the time five bolt HP289 engines started in production. The serial number 1 prototype 1965 six bolt engine was started in April 1963. This engine is covered in Bob's online version of his book. The engine was an Experimental High Performance 289 assembly. The engine block carried a cast in "SK-" engineering number. It also carried a group of hand stamped "XE-" prefix numbers which is how one would look up details in the engineering data files. GT40s and Cobras used five bolt block in severe international racing 1963-65. Cars were developed to race up to 48 hours between overhauls and many engines just got freshened and reused. I have a few Shelby engine shop dyno test multiple pages reports of used Cobra and GT40 race engines as part of being freshened up as required. Five bolt engine strength was not an issue to Ford or Shelby that I ever heard of.. A friend dirt track raced for years with a five bolt block. He lost more than one flywheel and or clutch (in a COBRA lettered steel housing) at 9,000 rpm plus and the block never broke. Shelby American did have problems with cast iron 289 crankshafts breaking, often at the main thrust bearing location. One of the Shelby team drivers told us at a SAAC convention that he noticed that his oil pressure had dropped during a race. He said when the engine shop dismantled the engine the crankshaft came out in two pieces. It had broken at an angle under the main bearing cap with the thrust bearing. The pieces could not get out or didn't until somebody dismantled the engine.