First of all there is not a darn thing wrong with a 305 Chevy! A 305 is every bit as good as a 350. I have built many examples of both. The 305 has a slightly smaller bore and this restricts the breathing capability in very high output engines. I would not worry about the 2 bolt mains, since even 375 horsepower 327 Corvettes were 2 bolt. The bottom end can handle up to 400 horsepower and still be reliable. Upgrade the rods if you are turning the engine to above 5000 RPM. Use GM PM rods or Eagles are good as well. The stock crank can be balanced just to be sure everything is smooth as is can be. High volume oil pump, NOT high pressure, HIGH VOLUME! Use a windage tray and good pan makes the oil system complete. There are some builders who are building "stroker" 305's which is about 330 CID depending on the bore. This is an even better choice if you budget will allow. I would upgrade the engine with VORTEC style heads, depending on your engine these should be available. A good Hydraulic Roller camshaft like an L31 350 uses will give you more power. GM Single Row truck timing chain hooks the crank to the camshaft, No Gear Drives since they trigger the engine Knock sensor and retard the timing! I would use KB Hyper Eutectic high compression pistons. High quality valve springs, better push rods, and roller rockers will help reduce valve train harmonics and friction. Back cut Stainless swirl polished valves are good to give a little extra flow. I would use heads with 170-180 cc intake ports and 50-60 cc combustion chambers and 1.91 intakes and 1.5 exhausts. There is a company called EQ which makes a head for this engine that is really good and not really expensive. The EQ head is built with very smooth ports which out flow many so called "ported" heads. These heads have screw in studs, thick decks, raised valve cover rails and other goodies included at a great price. Good free flowing exhaust, Full length headers, X pipe and Turbo or Flowmaster mufflers complete the package. I have done a lot of work to my original 305 from my El Camino, just because I wanted to keep the original engine in the car (numbers matching). A good TBI setup can produce up to 300 horsepower from the right combination. A TPI engine will make a little more power and a considerable amount of additional torque, just because that is what a TPI does best! It costs a little more to go with a 305 based engine, but it is fun to prove people wrong when they tell you that a "305 won't run"!
40 years as a Master Mechanic and Machinist!