Some more rhetoric, 2 bbl carbs and intakes were put on at the factory for one reason, to save money! The bean counters often ended up coming up with the packages, not the engineers!
I'd rate the 2 bbl as the major restriction on a 2 bbl engine, followed closely by a single exhaust - maybe the other way around depending on the engine. The intake and heads aren't very much different from the 4bbl components in flow capability, espcially if you can convert a set of #47's to the level of #48's with bigger valves and screw-in studs. Personally I wouldn't bother with the big valves in a 350 sized bore or the studs, because the added expense wouldn't add much performance. I've built and dyno'd enough engines for restricted 2 bbl and small 4 bbl classes to know how much each of the parts adds to the power, and I was very close to building a Pontiac 2 bbl 350 that would have killed the SBCs.
Incidentally, a switch from a 390 cfm Holley to a 750 on a 500 hp engine will add about 60 hp, and the NASCAR restrictor plate engines are down a lot more than that from the unrestricted engines!
My thoughts: change from a 2 bbl to a Q-jet, maybe step up a bit on the cam specs (an RV-type grind with about 210/215 @.050" and .420" - .440" lift), install a dual exhaust and decent mufflers and you'd go from about 250hp (on a very healthy 2 bbl 350) to about 280-290hp with no loss in low speed performance or drivability and very likely 1-2 mpg better fuel economy especially with the granny gears!
The only reason for the existence of 2 bbl set-ups with smaller valves, no screw-in studs and single exhausts systems was to provide a cheap alternative to a properly coordinated package. If the real goal was to increase fuel economy instead of just to cut costs, a better engine combo would have been a torquey V-8 built for under 4500 rpm operation with a vacuum secondary 4 barrel carb that has very efficient primaries, with slightly smaller intake runners and cylinder head ports, about 9.5:1 compression max for pump gas, mild cam, free flowing exhaust and tall gears - hey, that all sounds familiar!
Not to promote the Brand X engines' philosophy but very few 4 bbl SBC's received valves as big as the Pontiac 2 bbl engines. Only the top level performance engines got the 2.02" valves.
Performance expectations: the reality is sadly exactly what Tom has stated. When people put on a 4 bbl intake, better cam and exhaust system they have the expectation that they will be able to feel the difference. Properly set up with a mild coordinated package, the engine should make at the least the same if not slightly more torque in the lower ranges as before. The big improvement however is that the engine will be able to continue making good torque at higher revs than before. This doesn't change the seat-of-the-pants feel of acceleration that most people expect but it does improve the overall performance of the car.
