GM finally lifted it's ban on any engine over 400 ci not being able to be put into an intermediate body, and they stuffed a 455 ci engine in the bay for the 1970 model year. With the stage 1 option, horsepower reached levels of the rare and expensive 1969 Stage 2, but for a lot less. Sales increased closer to the levels of 1968, rebounding from the disastrous 1969 sales which dropped in half. Style was redesigned as well, and there was an option for a trim package, called the GSX.
1970 Buick GS Stage 1
For only $199, you could receive a hotter cam, larger valves and beefed-up springs, ported heads, and a better carburetor. Buick rated the horsepower at 360 for the Stage 1 (probably to help sell it, due to insurance premiums starting to increase dramatically for muscle cars), but most experts say that was too conservative, and it is more around 400 horsepower.
Motor Trend's testing at the time clocked a 1970 Buick GSX Stage 1 as the fastest American production car they had recorded up to that time. That would end up being Buick's best performance, since performance starting dropping in 1971 with new emission standards.
1970 Buick GSX
Mid model year, Buick released the GSX package, which was mostly cosmetic options. The only two color options were Apollo White or Saturn Yellow (America was buzzing about landing on the moon just the year before), and a stripe pattern with unique spoilers was put on. A tach mounted on the hood, a four speed Hurst shifter, front disks, and a better suspension.