Bear, Tex Johnson did more of an aileron roll than a barrel roll.
YouTube - Boeing 707 roll by Test Pilot Tex Johnson If you will observe, the aircraft in the video is on more of a ballistic trajectory during the maneuver.
From memory, the type of barrel roll you do depends a lot on airspeed. We used to do 90 off heading barrel rolls in the T-34 and other slow aircraft and 60 or 45 degree off heading barrel rolls in various tactical jets. By off heading, I mean how far you were off initial heading as the nose fell through the horizon inverted. Roll rate and g are not constant although g is always positive if done properly. Not a terribly difficult maneuver.