On my 455, I switched from the 3.73's I had in the rear to 3.36's and the car tended to pick up some. If your current combo is throwing all the torque at you very early and falling on it's face without many revs, those 3.73's are just helping you blow past the torque the engine is making. Taller gears can help you spend more time in the engine's power band, turning it into speed.
I also agree that the intake and carb are likely too small. I would switch to either a factory intake manifold or to a torker ii. It's unlikely that the Performer RPM will fit underneath the hood of your 67 firebird. It's a big engine, the single plane manifold isn't going to kill the torque.
If you're set on changing the cam while you're in there, I think pretty much everyone would lean towards that crower 60919. If you need more manners out of it, you can pair it with a set of Rhoads lifters. Just make sure that the cam and lifters are checked specifically for proper taper, runout and hardness. There's been a lot of testing on these through 2023 and hardness doesn't appear to be an issue, but cams ground with no, or improper amounts of taper as well as lifters with out of spec face machining.
If the engine has been line bored, do you know how much it took to get everything straight? If not, you may need to contact the builder and try to determine, otherwise you're guessing. Either way, try to get a timing set that uses an IWIS chain.
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-Jason
1969 Pontiac Firebird
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