Upper Image is 70-77 model originals:
Length across top: 22 cm. Length across bottom: 21.5cm. Height total: 5.7cm. Hole height combined: 3.75cm. Hole width: 9.8cm
Lower Image is 78-81 model originals: Length across top: 22 cm. Length across bottom: 21.2cm Height total: 6.5cm. Hole height combined: 5cm. Hole width: 10.7cm
For the two series of dash/radio's that are commonly found on LH & RH Drive 70-77 & 77-78 Pontiac Firebird & Trans am
Click on the image to enlarge it.
The lower image is the actual 78- 81. but due to many factors, the conversions often have been done with a standard template leaving this, who knows, hence the measurements we have shown here to help.
ALTERNATIVELY: People putting in aftermarket radio systems may be best to decide whichever of these best suits.
Nice part; original chrome edges and detail as shown.