1st to be modified specifically to set a record (1956 Boneville Salt Flats, 187.992)
1st small block with a GMC blower
These are not yet verified, and may never be, but it's pretty certain that with no documentation from the 1930's through 1970's like the way everything is documented now, it'll never be proven wrong even if it's never proven correct.
The engine? A prototype 265 cu in given to Stu Hilborn by GM in 1954, then sold to Tom Cobbs and installed in his 29 Ford on 34 rails