You might look at this site for more info...
B-58 Hustler Association HomePage
This site also has good info:
Convair B-58 Hustler
From the second site:
Several accidents had revealed that the Convair-developed ejection seats were not sufficient to protect the crew throughout the B-58's performance envelope. Consequently, an encapsulated seat built by Stanley Aviation Corporation of Denver, Colorado was adopted.
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The B-58 accident rate in 1959 and 1960 had been alarmingly high, which led SAC to delay acceptance of executive responsibility for the aircraft.
The first accident had taken place on Dec 16, 1958, near Cannon AFB, NM when 58-018 was lost. The accident was attributed to a loss of control during normal flight when autotrim and ratio changer were rendered inoperative due to an electrical system failure.
On May 14, 1959, 58-1012 was destroyed by fire during a refueling operation at Carswell AFB.
58-1017 was destroyed on September 16 of that year when a tire blew during takeoff from Carswell AFB.
On October 27, 55-669 was destroyed near Hattiesburg, Mississippi when it lost control during normal flight.
On November 7, 55-664 was destroyed during a high-speed test flight near Lawton, Oklahoma when it disintegrated in midair. Convair test pilot Raymond Fitzgerald and Convair flight engineer Donald A. Siedhof were both killed. The flight was attempting to collect vertical fin side loads data under the conditions of the loss of an engine at high speed. A friend of mine witnessed this accident from the ground. Although the cause of the accident was never adequately explained, it appears that a design flaw in the aircraft's flight control system and defects in the integrity of the vertical fin structure were to blame. There is also the possibility that when the number 4 engine was purposely shut down for the test, number 3 lost thrust as well.
On April 22, 1960 a failure of the Mach/airspeed/air data system caused the loss of 58-1023 near Hill AFB, Utah. On June 4, 1960, 55-0667 was lost due to pilot error while flying at supersonic speed near Lubbock, Texas.
The unusually high accident rate made SAC apprehensive about the reliability of the aircraft in service, and led to postponement of Category III testing. In addition, the Fitzgerald accident raised questions about certain aspects of the control system. As a result, B-58s were restricted to subsonic flight only for nearly a year afterwards until the control system and tail structure could be fixed.
This site discusses the ejection capsules:
http://www.ejectionsite.com/eb58caps.htm
The USAF says this:
Factsheets : B-58 Escape Capsule
When the supersonic B-58 Hustler entered service in 1961, it had individual ejection seats for its three crew members. However, ejection at speeds above 665 mph was extremely hazardous. To improve ejection survivability, the Stanley Aircraft Corp. developed a high-speed high-altitude capsule ejection system that would allow safe ejection at supersonic speed. The capsule was adopted for retrofit beginning in late 1962, making the B-58 the first USAF aircraft with a capsule ejection system. It was effective throughout the flight envelope up to 70,000 feet and twice the speed of sound.