Re: 396/375 or 396/350
All you have to do is start a cold L78 and listen to the engine warm up to make your decision. If you have a soul, the staccato exhaust and solid lifters will speak to it.
The comments from the Chevelle owner are accurate and bear in mind that the Camaro is the lighter car.
I owned a L78 equipped 67 and they work pretty well, even as a daily driver (which mine was). Use premium fuel and give serious consideration to upgrading the brake system (not just to a sliding caliper design, but 4 piston).
As for the weight, get a big front sway bar, good gas pressure front shocks, Hotchkiss BB front lowering springs, consider making the car an L89 (I did) and decent modern tires.