I've got a '78 Camaro with the 250 L6 and 3spd manual. The pre-smog (depending where you draw that line, as different emissions controls have been introduced over time since '67) motor of '74 and earlier had a completely different top end, as the integrated cylinder head and intake manifold was introduced along with the catalytic converter in '75. So you would need to swap the entire top end out to create an earlier 250; cylinder head, valve cover, intake manifold and exhaust manifold. The exhaust manifold is also a 2" outlet, versus the 2.25" '75 to '78 outlet, and exits at an angle on the driver side instead of downward and around the oil pan to go back the passenger side. So the '75 to '78 exhaust pipes will not fit the earlier exhaust manifold.
However, if all you're looking to do is take off the EGR, EFE, and catalytic converter. You can probably just disconnect them, cap off the vacuum ports, and put a block off plate over the manifold hole for the EGR if you remove it. I'm not sure if you are looking to replace the mono jet with an earlier one without the adjustment caps, or rebuild it with all the fixed factory set screws exposed? I haven't desmogged my Mono-jet. The Thermac air cleaner has been on the 250 quite a way back and has a ducted hose to let cooler air in, so I'm not sure if you'd want to remove that. There is also the PCV which should be left on for crankcase circulation. The EVAP canister is different from the small block as it has an extra valve on it hooked into the EGR system. So I don't know how disconnecting the EGR would effect it? I'll post some more ideas with pics for you when I have time later.
