"The standard transmission in 1958 was a manual column-shift three-speed. Two optional automatic transmissions were also offered, a Turboglide and a Powerglide at $231 and $188, respectively. Two manual transmissions were also offered: a standard three-speed and an
optional BorgWarner four-speed. Both had an iron main case and tailshaft housings. A Positraction differential was also available for $48."
The above quote was from a 1958 Chevrolet Internet site.
The following is a partial article that appeared in Old Cars Weekly back on February 3, 2016. The article is about a 1958 Delray with a four-speed transmission that is owned by a friend of mine that lives about 40 miles from me. The car has undergone extensive judging and it has won several awards.
"It’s long been believed that the first year for a full-size Chevrolet with a factory four-speed transmission was 1959. However, some late-production ’58s left the factory with four-speed transmissions. They were conversions, but conversions done at the assembly plant nonetheless. As such, these four-speed, full-size 1958 Chevrolets should be considered factory-installed.
Regular Production Option (RPO)No. 685 in the 1958 passenger car instruction manual dated Feb. 13, 1958, shows the four-speed conversion on four pages. The conversion included a unique transmission and extension assembly, a floor-mounted control lever and bracket, specific control rods and adjustment; a four-speed transmission mounting; and a seal and retainer for the floor-mounted shift handle.
The steering column for the three-speed manual transmission was left in place with instructions on locking the column’s internals and gluing the lever grommet in place so it would not fall off after the lever was removed. A “transmission control selector” instruction folder was to be placed by the factory over the sun visor to inform the owner about the four-speed transmission. The 1959 Chevrolet parts book also contains many part listings for the 1958 passenger car four-speed transmission from group 4.004 thru 4.103."
To view the complete article on this car with the documented four-speed transmission, along with photos, go here:
https://www.oldcarsweekly.com/car-of-the-week/car-of-the-week-1958-chevrolet-delray 
