If your talking the spring on a power window it helps raise the window when you activate the power window so it takes some stress off the motor.
Any size SBC engine will fit and is compatible with a 91 Firebird. A 350 engine is the most common that is used for installing in a 91 Firebird.
I calc'ed my '91 V8 Formula Firebird to be about 18 mpg.
A firebird can't "have" third gen. It either is or is not a third generation firebird, and Yes, a 1991 is a third generation.
700r4
on the transmission tailshaft housing
Best way to do it is either a flatbed or a trailer
it should be on the door lock motor on some early '91 models
There are 91 days of Spring in the Southern Hemisphere.
Disconnect wires at antenna
yes it is fuel injected.
91-93 octane premium
There are no adjustments on the rear wheels, so no.