Darryl,

Was hoping this radio would fit my brother in law's 75 Chevelle..unfortunately it doesn't.

While searching the numbers,discovered GM radio identity is based not only with the model number but also with the

service reference number....(your radio - 90BPBT1) is the following:

From the 1979 Delco Service index 27D-1979-2, 90BPBT1 is a Corporate model:

1979 GM Corporate AM radio, 8-track tape. Corporate meaning 90BPBT1 application was for many models: 

Buick Skyhawk, Skylark

Chevrolet Malibu, Impala, Caprice, Camaro, Monza, Nova.

Oldsmobile Cutlass, Delta 88, Custom-Cruiser, Ninety-Eight, Toronado, Starfire, Omega.

Pontiac Lemans, Catalina, Bonneville, Firebird, Sunbird. 

Trucks.

For the older radios,1968 to 1975 the Service Reference # is almost the same,but the numbers represent a different car (older) of radios.

http://wildaboutcarsonline.com/cgi-bin/pub9990262549620.cgi?itemid=9990346167137&action=viewad&categoryid=9990346166551

Interesting to note that the following "ultimate" radio to buy from 68-75 was:

The ultimate in stackable units was reached in the late 1970s with the model "(part number) + BFTC1"

which was an AM/FM w/Stereo and 8 track and CB radio unit in three pieces -

the radio unit which went in the normal dash position, a CB unit which contained the transceiver electronics,

and a control head/microphone. This was about as much as was ever put into a Delco radio.