The original 4-speeds are known as the "SM420" with a Granny Gear first gear and 1:1 4th gear ratio, they are almost bullet proof. The Small Block Chevy (SBC) V8 engine was optional in 1955, so your 305 SBC will bolt in. It won't give you any real speed inprovement (3000 RPM is still 3000 RPM regardless of what engine is turning it), but will give you more power. You will also need a deep fan shroud.
I have a 1956 8400 that started life with the 265 V8 and now has a 350 SBC V8.
Mike B
