You still left out a lot of information like your transmission type, desired power level, type of performance, and desired fuel.
If you're not sure what to get, then your best bet is to hold off on the engine swap. It might be best to put in some practice time on your 305 to both get things in order for the future, and to help you figure out what you will be happy with.
Your 305 will take many of the same performance upgrades as any other generation 1 Chevy small-block, such as: Headers & exhaust, intake manifold, carburetor, fuel system, ignition system, torque converter or gears, retro- hydraulic roller camshaft & lifters, and some others.
If fuel mileage is a concern for a daily driver, then you might want to plan to run on 87 octane from the start, and avoid a couple of gas mileage killing modifications like large camshaft and your rear gear swap.
A rear gear swap isn't mandatory, and not necessarily best for a daily driver. You may be happier by swapping to a good quality torque 9.5" converter that will increase acceleration without the need for a rear gear swap. If you stay with a lock-up style converter, your fuel mileage won't go down.
You can also get a way with going a slight bit large on some other parts for your 305 like an Edelbrock RPM/airgap or similar design intake manifold, and investing in a quality set of stainless-steel 1.75" primary tube exhaust headers - (shorties are easier). These parts will work to boost power on your 305 and very easily swap to a 350 or 383/400 in the future.
If you don't know much about building engines, then you might want to consider buying a crate motor in the future. Just watch out for the cheap ones. If you have an honest motor machine shop in your area you may want to contract them to build your longblock to ensure good quality.
The matching of performance parts for this theoretical engine will depend greatly on all of those factors I mentioned earlier - and what your choices for gears and usage will be.
For instance, if you stick with a carburetor and up to 383 cubic inches for daily driver(DD), then you'll want to limit the intake port volume of the cylinder heads to no more than 180cc.
On a DD you will want to keep the camshaft intake duration in the 205* to 215* @ 0.050" range. Then you won't be required to increase mechanical compression ratio to the point of requiring more than 87 octane fuel.
If you are looking at aftermarket heads, then you want the flow ratings on your heads to be fairly close inline with the valve lift provided by your camshaft choice. If your cam only has 0.480" lift, then you don't need heads that flow great at 0.600" lift. For a DD you want heads with high swirl that have very good flow at low valve lifts. Mildly ported factory iron heads work well for DD's.
I'm assuming daily driver because that describes many of cars on this forum. Many of them are just weekend cars that are built for more power and sacrifice some ease of drivability and a good bit of fuel mileage - often requiring premium fuel.
If you're building a weekend car that will be carbureted and still automatic trans, AFR - Air Flow Research makes an ideal head. You'll still be looking for 180cc intake runner, small combustion chambers (64cc or less), and 23* valve angle. They aren't cheap, but they have great power potential.
A weekend car often has both a high stall torque converter, a built transmission with cooler & shift kit, and more rear gear ratio. With these added stall & gears your range of camshaft duration increases to anything from 220* to 240* @ 0.050" intake duration. Valve lift with the AFR heads would need to be around the 0.600" range. These larger camshafts will require higher mechanical compression ratio in the 11:1 - 12:1 range (with aluminum heads) in most cases, and require premium fuel or better.
The Edelbrock RPM/airgap style of intake manifold will still perform well on a weekend car with an automatic trans.
For a very fast DD that performs as well as most weekend cars, you might want to consider building a 383 stroker with a kit, or buy a crate version - shortblock, and add your own heads & intake. The AFR-180 heads and the RPM/airgap intake manifold, + 1.75" headers mentioned above work well on a 383. If you stick with mild (smaller) camshaft, the 383 will not require more rear gear or higher compression ratio. By milder cam I mean low on intake duration (215* range), but high on lift (0.500"+) to take advantage of the AFR's flow capability. With compression limited to 9.5:1 - 10.0:1 a 383 will run on 87 octane.
There's some starter ideas. When what you want becomes more clear, it will be easier to make more specific recommendations on a working combination. For now I'd say play with the 305 for a while by adding some better parts that can still be used on future engines.
IRT your questions:
1. I like the LPE 74216 hydraulic roller camshaft. It works very well for 305 - 383 engines with carburetors and automatic trans. It is 100% streetable, but requires retro-fit hydraulic roller lifters and a timing chain/gear with a cam button.
http://www.lingenfelter.com/mm5/merc...egory_Code=C30
2. Ported stock heads - LB9,L98,LT1, ported SR/world products, AFR heads, TFS, Canfield, Ported Edelbrock heads, - and several others. Your final combination and budget would dictate the choice and level of porting/modification for each type of head. The porting levels and parts combinations for cylinder heads is endless. AFR 180's are great out of box heads - can't go wrong.
3. You do not need a double roller timing chain. A true roller type of chain design is superior to stock style chains, but you only need a single roller. If you convert to roller cam & lifters, then the timing gear has to match your cam type and have provision for a cam button to keep the camshaft from walking forward.
