305:
-Port stock heads
-Port manifolds
http://www.pirate4x4.com/tech/billav...0Abrasives.pdf

(The above link is the instructions on how to port a 305 head using the standard abrasive deluxe porting kit. I would also look at buy a carbide too, but if it's your first time, it's hard to totally damage your heads with that kit as it's just sandpaper rolls.)

-Install a single pattern 268 cam on 110LSA. Call Lunati and order part number DV268-10 HRH AND MAKE SURE YOU TELL THEM YOU NEED IT FOR A POST 1987 FACTORY HYDRAULIC ROLLER BLOCK WITH THE APPROPRIATE CAM SNOUT TO ACCOMMODATE THE THRUST PLATE! (This is actually David Vizard's cam spec from his how to make Max Performance Chevy Small Blocks on a budget)
-COMP MAGNUM 1.6 Roller Tip rockers (check piston to valve clearance before reassembly!)
-Hooker Shorty Headers
-RANDOM CATS if you are running them, otherwise just get new Walkers if you're on a budget.
-Bump fuel pressure by either using washers underneath the fuel pressure cap and using a fuel pressure gauge OR buy an adjustable fuel pressure regulator. 50psi is where you want to be.
-Computer tune (Call PCMforless if you aren't doing it yourself and let them know you bumped fuel pressure up to 50psi)
=From stock 190/210HP to non-stock 250-260 HP
(Warning-you actually may exceed the capability of the stock 19lb. injectors. If it constantly runs lean, it means you need new injectors and another computer tune. The 50psi bumps HP capability of injectors to 260HP from stock 241HP capability at stock 43.5psi)

350:
Same, but:
-Cam should be DV268-08HRH Call Lunati and order part number DV268-10 HRH AND MAKE SURE YOU TELL THEM YOU NEED IT FOR A POST 1987 FACTORY HYDRAULIC ROLLER BLOCK WITH THE APPROPRIATE CAM SNOUT TO ACCOMMODATE THE THRUST PLATE! (This is actually David Vizard's cam spec from his how to make Max Performance Chevy Small Blocks on a budget)
-Runners need to be the AS&M or TPIS big mouth 1.66" diameter Runners to work with that cam. This provides the "Tuned" ram charging of the TPI runners. Anything smaller does not work unless you want to go with a 290 Cam-which is too big for the street and sucks on gas mileage.
-You actually may need to upgrade injectors on a 350 because even though you boost the psi of the fuel pressure regulator to 50, it only allows you 300HP. The above combo hits actually hits around 320-340HP. If you stick with the stock runners, you'll probably be around 300 HP and may be ok to use the stock injectors. But southbay fuel injection or Injectors Plus sells a complete set of rebuilt 30lb. injectors for CHEAP and with a warranty! I went with Injector Plus 36lbs and they are bosch III injectors on my 383.
(Notice how the runners add about $500 to the bill. The stock runners were tuned to a 305, not a 350.)
-If you want more, you're going to have to go with AFR heads and possibly the Hooker Super Competition full length headers and six inch rods. (Think 400HP)

If you're going the 383 route, you can and should go with a FIRST fuel intake system and a DV268-06HRH HRH AND MAKE SURE YOU TELL THEM YOU NEED IT FOR A POST 1987 FACTORY HYDRAULIC ROLLER BLOCK WITH THE APPROPRIATE CAM SNOUT TO ACCOMMODATE THE THRUST PLATE!

You should hit 420-450HP no problem with 36 lb. fuel injectors and ported heads. You don't even have to port the intake or runners as they are HUGE as cast and made to fit a 383. If you want 500HP just go with JE SRP pistons or CP Bullet pistons and Vortec style chambered heads or AFR's. And try to get close to 10:1 compression ratio if using cast iron, at the least 9:6, or if aluminum, 10:5.

That's the 383 build I should have built in hindsight.

I built a 383 Super ram with the 270-06HRH cam with Icon pistons giving me 9:5 compression ratio and am just waiting until the spring/summer to dyno it. Used Vortec heads and Dyno Don headers.

Otherwise, yea, you can buy any of the newer off the shelf LS motors. It would still require machining.

Just depends on how much $$$$ You want to spend. Speed cost money, how fast do you want to go?