Ford offered the Cobra Jet engine for the first time in 1968 1/2. Carroll Shelby, always one to up the competition, be it Ford or GM, put the 428CJ engine in his GT500 Shelby Mustangs about two thirds into the model year. It was cheap horsepower. Early '68 GT500's were given Ford's 428 police interceptor engine. Rated at 360 hp at 5400 rpm, the big block produced 420 ft. Lbs. Of torque at 3200 rpm. Mid-year, Shelby got wind that GM intended to introduce a 396 Camaro and call it "King of the Road." Shelby beat them to it. Before GM could follow through, Shelby American offered a GT500KR, King of the Road, and stuffed the Cobra Jet engine under the hood.
The '68 GT500KR was a very special car. Many modifications were made to the stock GT Mustang. The 428 was the same Cobra Jet engine offered in the stock big block Mustang. It was the old 428 block with a lot of changes. The low riser, revised, 427 heads had huge rectangular ports. Measuring 2.34" x 1.34", the ports were larger than the 427 racing heads. A special dual-plane intake manifold held the mammoth Holley carb. Stronger connecting rods and crankshaft were replaced the stock 428 pieces. Dished alloy pistons were used with eyebrows, attached to con rods with full floating pins locked in with two retainers. This lowered the compression ratio to 10.7 to 1. Pop up pistons were available for drag racers. The KRs all received heavy duty front and rear shocks, the four speed cars got staggered rear shocks, power front disc brakes and power steering and big capacity rear drum brakes. Ford did not promote the increase in horse power the Cobra Jet engine had. It was advertised at 335 horse. That figure was off at least 20 percent. Remember during those years the insurance companies had started to penalize high horsepower cars. In fact, these years were the beginning of the end of cars like the gt 500 KR.
--from carmemories.com site
I seem to remember they made just about as many KR's as they did standard GT-500's in 1968. I think the KR's had a ram-air deal, also. Mr. Mustang can fill in the details.
When I drag raced in 1968, my GT-350 never lost to a KR; we considered them overweight "sleds"
(...back in my small-block days
)