Depending on the year, it could have a 301,(326 unsure),350,400,403
Olds,or a 455. If it's a plain jane and has a V-8 then it probably has
a run of the mill 350ci.  I think for myself that it would be worth it
no matter what engine came in the car.  Most parts can be found as OEM
or Re-pops.  It just takes the cash and a will to finish it.

                         ...Ron
--
68'RS Camaro
88'Formula
00'GT Mustang

If you look at the VIN it will tell you the model and engine.  For late 70's
vintage, 77 to 81 which would have T-Tops, unless you have the rare 76
Limited Edition.

For example:

2W87W9L000000
2 indicates Pontiac.  (Note, 81 VINs are preceeded by 1G2 instead of 2)
W87 indicates a Trans Am.  Other models are as follows: S87 is a base
Firebird, T87 is an Esprit, and U87 is a Formula.
W indicates the engine, a 301 in this example.  The 400 is a code Z while
the 403 is a K.  L, R, or X is a 350.  Y was also used for the 301.  G and H
were Chevy 305's, and A or C were 6 cylinder models.
9 is the last digit of the model year, in this case '79.  For 1980 and 1981
this would be A or B respectively.
L is for the production facility that built the car.  L is for Van Nuys, CA
while N was for Norwood, OH.
The last 6 digits are the sequential production numbers.

If you have a Z code you need to look at the block to see if it is a W72
400, the highest horsepower model, unless it is a '79 where all 400's were
W-72's.

I had a 79 as I recall there were 2 engines gm400 was popular but
pontiac ran out of engines and replaced them with the olds. 403 that is
what mine had

T-Tops were available in the 1976 50th Aniversary T/A(limited production) and
all starting in 1977 so mostly likely the car is of the 77-81 years.  1977
was the first year GM began widely use "corporate" engines.  In a Firebird
from 1977-79 you could get either a Buick 231 V6, 305 Chebbie, or a Pontiac
301 or 400.  From 80-81 the Buick and Chebbie were still avaible but the
Pontiac 400 was dropped and replaced with the Turbo 301 and a 265 cu. in.
version of the 301 was introduced.

The only GM 6.2L that I know of is the 6.2L V8 Diesel used in trucks.