In '69, a few 350HOs DID come with 48s. I've CC'd a couple sets and their chamber was a bit smaller than the 400, coming in at 68. We've found the 350 "likes" the small valve heads better, as there's not enough bore to make the larger intake valve "work". I've also seen where too much lift runs the intake valve into the top of the block (bad).
350 Pontiac was never a "me too" engine (introduced at the beginning of the '67 models). Is has the capability of far more low-end torque and not as much high-end horsepower as 350 Chevy (apples to donuts). '69 Z/28s had 302. '69 T/As had 400 Ram Air III or Ram Air IV engines, and the little Chevy had no chance against them. The "race" version of T/A had a 303 CID Pontiac based on the 400 block (4 1/8" bore) and a 2 7/8" stroke, with Ram Air V "tunnel port" heads on them. The 302 had a BIG advantage until someone took the Vs off and put Ram Air IV heads on. The little Pontiac REALLY came alive then, but didn't win anything until Chevy and Ford quit putting money in the Trans-Am (Trans-American Challenge Cup) series. Pontiac didn't put money in it (no factory "sponsorship") except to develope the "legal" displacement (5.0 lr. max). It wasn't 'til '75 that Jerry Titus ran the Radial T/A "Tirebird" to a championship. The same was true of the '69 Camaro SS350 against the Firebird "400". Granted, the Firebird came with a bigger engine. The 350 cars were a "downgrade" from the 400 cars. No one I know ever tried to REALLY race the 350 against the small blocks "heads up". We DO have guys doing well in NHRA "Stock" with 350 Firebirds. They're "factored" lower than the 350 Chevy, as it (the Chevy) is better at making horsepower. NHRA doesn't seem to pay much attention to "torque".