Nacho,
There's a "partial VIN" stamped on the front of the block, just under the passenger's side cylinder head. There's also a two letter "code" there, identifying WHICH engine it is.
Pontiac Historic Services (PHS) will provide you a "build sheet" for a nominal fee and the VIN. That will tell you everything the factory put in the car.
The transmission is not a PowerGlide. It's called "Super Turbine 300", and the only similarity to PG is it's a 2-speed. For performance, it's a dog. TH350 bolts right in where it was.
The value of these cars is pretty high, but nowhere near what a "400" car brings. I would guess in the $5-7K range.
The best part is, ALL of the "good" Pontiac engines will bolt right in without modification. You can take the original stuff out and stash it. Build a "fun" one, and if the urge strikes you, you can put it all back "original" and sell it for max value.
There are several disk-brake conversions for the 1st gen F-body. We like the Master Power system because it most closely resembles the factory Delco/Morraine system found on the '69 models. Bolts in, works GREAT!
Jim