Not a whole lot. I would focus your spending on an aftermarket intake manifold, exhaust, ingition system, and mabye some underdrive pulleys. All of these things could be used on a 350 down the road. It just isnt worth the money to try to build up a 305.
Go with a dual plenum street intake like a Performer, and with a 305, I wouldnt go with a larger carb than a 500 cfm. You could pick up about 15 hp with this through mid and top end.
A good exhaust system will do wonders as well. What goes in has to get out as well. With headers and a good exhaust, you could stand to pick up another 10-15 hp. Looking at a gain of 25-30 hp, and that would be noticeable in the seat of the pants, especially in a light vehicle.
I would guess around 20 - 25 hp with an aftermarket intake and carb. Again, if you are planning on using these parts on a 350 in the future, the parts you choose will be a little large for a 305. They will still work fine, but they will not produce the optimum performance on a 305. I would look at a performer or Performer RPM intake and a 600- 650 holley carb. The Performer intake and 600 carb would match your current combo better while the performer RPM and 650 would match a 350 better.
With that displacement engine, and knowing you wont be able to turn it high rpm, going with a whizz bang double throwdown double pumper 750 or such would be a waste of money. Your engine couldnt use all that carb. Not to mention that it could actually hurt your performance.
To select the proper carburetion needed use this formula.
CID X RPM divided by 3456. This will give you the required CFM at 100 % efficiency.
305 X 5000 = 1525000 divided by 3456 = 441 CFM.
350 X 6000= 2100000 divided by 3456= 607 CFM.
As you can see, even if you want later to go to a 350, that unless you are going to install a hot street cam and be able to turn over 6000 rpm with it, going with a larger carb isnt necessary.
Remember, the NASCAR guys are running 390 CFM carbs with a restricter plate under that which limits the air intake even more and they are getting 600 hp out of those engines and turning 7000 rpm.
With either engine, unless I was going to put a hot cam in it and turn it over 5000 rpm on a regular basis, I would opt for a carb in the 500 CFM range.
In case you are wondering what the 3456 is in referance to, that is the cubic inches in 2 cubic feet. Since a 4 cycle engines cylinders only breath in on every other revolution, it only requires half of what it would need were it a two stroke engine.
I would stay with the Performer type intake on both engines, as its rated from 0-6000 rpm where the RPM series works in the 1500-6500 rpm range. Going with a higher rpm intake on a stock engine would not help you at all. What is the use of this engine?
Is it in a pickup or passenger car and what is your driving style?
Most street driven vehicles very seldom see over 4-5000 rpm under normal conditions, so look at it realistically as to how its going to be driven.
I have run a 600 CFM carb with a Performer on a 351W with Twisted Wedge heads, and have turned the engine over 7000 RPM, and it still had room for more. This on pump gas with 9.4:1 compression, and a 230/235 solid lifter cam with 510/515 lift.
You can always wind an engine tighter (til the valve train or something else gives way) but you cant underwind it if its over cammed or over carbed.
MI 2600, thats a good point I missed. If I remember correctly, the 1.94 intake is as big as the 305's bore will handle.
You could probably do just as well with a set of 1.94 intake /1.60 exhaust valves and a good bowl job and 3 angle valve grind. That, along with the 4 bbl and good exhaust would probably be worth another 30-35 hp, even with the stock cam. And all the power would be within the current power curve of your engine.
If you decided to go on to a 350 at a later date, you could easily put your worked over heads on it as well. You wouldnt see any appreciable gains running the 2.02 intakes unless you are going to be turning over 7000 rpm anyway.