Your 1969 has, or at least originally had, a Pontiac 350. Chevy, Buick, Olds, Pontiac, and Cadillac all made their own engines for a while, think Pontiac dropped theirs in the late 1970s to start using GM standard (Chevy). Olds and Buick dropped theirs around the same time. The Pontiac 350, 400, and 455 were all the same block, and, yes, I have heard them termed "big blocks", but they are not the same as the Chevy big-block engines. Pontiac 350 has a 3.875 bore and 3.75 stroke (actually is 354 CID), their 400 is a 4.125 bore and 3.75 stroke but not interchangeable with Chevy 400. Totally different engines. Olds and Buick had their own 350s and 455s also, again not at all the same engine as the Chevy 454 or Poncho 455. Olds had a 403 also. Olds engines had big bore and short stroke for highway cruising, Buicks and Pontiacs more "square", and Chevy kind of in between. there were smaller engines--Pontiac 265 and 301, Olds 307, etc., and I'm not sure whether they were the same blocks or smaller ones. Am thinking the Poncho 265-301 were "small" blocks, as compared to the 350-400-455. The "Rocket" was Oldsmobile, not Pontiac. Hope this helps. Disclaimer--it's been years since I played with GMs, had two '74 Pontiacs and at one time was pretty familiar with them, but memory turns gray faster than hair, ya know.

And of course, that Firebird is 35 years old now. It may or may not have the original engine, could have been through several by now, and not at all unusual to find Chevy engines in older Firebirds, because parts are much easier to find for Chevies than the old Ponchos.