Great question. Karol Miller has the most recognized speed record, but I'm sure there are other Y-Blocks that have gone faster in coupes or roadsters. A colleague and mentor tells me of the days when he used to race a Y-Block in a 34 coupe. He said they did something north of 170mph.
That is fast, but I believe there were probably even faster cars. I talked to a youngster last year who was building a 32 roadster with a 671 blown Y and his goal was 200. His father has landspeed experience and I don't doubt they will give it a good try.
Of course, there are no engine designations on the records set at landspeed events, so it would be hard to verify. At any rate, it would be great to hear if anyone out there has reliable info on previously fast Y-Blocks (don't forget Australia).
Charlie Burns Laton, Ca (South of Fresno)

BurnsRacing981@gmail.com