The few known today are suffix J (casting number behind intake) blocks if I recall correctly. Last fall I bought the original camshaft,
oil pan, and valve covers removed from a Cobra HP260 engine by the original buyer decades ago. He even sent me the original Ford serial number for the block (the HP260s were numbered by Ford like they subsequently did five bolt HP289s). Cobras weren't the only recipients of off line built HP260s by Ford. Apparently Holman-Moody got some and Dearborn Steel Tube got some for test mule cars. At SAAC-31 a man tried to sell me one with a Holman-Moody "bird" intake that was supposed to have come out of a European rally car. Based on the engine serial number of a still running engine I got parts from and research last winter there must have been about 200 engines made that went someplace.
Block wise we believe there was nothing special. The special parts known to date were intakes, connecting rods, cylinder heads, camshafts, and distributors. I have examined one car's XE rods and cylinder heads. I have the camshaft for one engine and seen the distributor from another.