Went just the opposite of the way you are going. When I purchased my '69RS (Base 327/210 Hp) coupe in Nov 1991, it had the incorrect 327 engine in it. The intake manifold on the engine was #3919803 with a 4BBl Rochester QJet. As mentioned above, this intake was for 1967-68 327 CID engine, 300/350 Hp. I recently sold it to another 68 Camaro driver who is doing the same thing that you are doing - 2 BBl to 4 BBl conversion.
If the heads for '68 327/210 Hp engine are similar to the 3927185 heads used for '69 (and they probably are), the valve sizes are 1.74" dia. intake, 1.54" exhaust. Although the 4Bbl "803" intake will have more capacity, the heads will still be the flow restriction and I don't believe that you will get the extra punch that you are hoping for.
For my car, I did the exact opposite - originality was key to me, so I wanted the 2BBl setup. The '69 327/210 2BBl engine that is in my car now runs and performs better than the engine originally in the car because everything works well together. When I had the correct '69 327 engine rebuilt in 2005, I enlarged the intake valves in the "175" stock heads from 1.74" dia. to 1.94" dia. and installed a slightly different cam to help the low end. Engine has very good low end torque and response. 2 Bbl just dies about 4200 rpm. But then I'm not going to run the car that hard. With the Powerglide and 2.73:1 rear end, cruising a 70 mph is about 2400 rpm - and the car goes 0-60 reasonably well.
If you install the "803" intake or any similar intake, please note the post below from JohnZ regarding this type of mainfold. The "803" intake that I sold had the hot slot.
Re: Q-Jet swap problem
« Reply #1 on: 07 January 2006, 09:52:55 »
Can't help you with the mismatch issue, but if your intake has the "hot-slot" like the one in the photo below (that's an intake for a Holley, but the same "hot-slot" design was used on many cast-iron Q-Jet intakes), I recommend plugging the holes at each end of the slot with core plugs as shown in the photo. That slot is connected to the exhaust crossover passage, and allows hot exhaust gases to heat the carb base at cold start for improved atomization of the air-fuel mixture, with a stainless steel heat baffle to protect the carb base from direct impingement of the hot gases. Unfortunately, that heat also fries/distorts the carb base and can cause the bottom well plugs to fall out of the Q-Jet float bowl, resulting in engine fires. GM recalled millions of Q-Jet-equipped cars with this "hot-slot" design in 1969 due to the fire problem, and abandoned the "hot-slot" design in 1970.