Why not by pass Old Version and go LS. 5.3 junkyard engines or good ones for less than 1000.00 adding a cam and you have 400+hp. Another 500.00 - 1000.00 for all the peripherals to get it to installed.I have seen that most of the answers to the questions regarding performance modifications on a 305 is "build a 350 instead". Granted, I will agree that using comparable parts a 350 will always make more power.
All of the modifications that work on any other engine will work on the 305 as it is nothing more than a typical internal combustion engine. It might not make as much power as some other combination, and all engines have some sort of limitation as to what you can achieve. Vortec heads will work on a 305, but in order to get the compression back up, you have to mill them a whole bunch making them essentially useless on anything other than a 305. Not to mention that the chamber "width" is larger than the bore of the 305 creating a "step" in the airflow into the cylinder potentially hurting performance further. Pistons and rings might cost a bit more than a 350, but they are still not anything "Oh my god."
307's may have made more advertised hp, but they were also "gross rated" an not "net" rated like the 305. They (305) typically are not any more or less durable than any other small block Chevy if assembled correctly. Everybody has a "magical combination" for the 350 and claim that it will "beat anything on the road" in some form or another. If it was pure performance that everyone was after, then build a bigger engine. For reasonable street performance, the 305 is not as bad as some may claim. The first engine I ever built was a 305 in a '78 Nova with a four speed. It ran high 13's at just under 100 mph and that was with a peg leg 3.08, and 225/60-14 radials. Trust me the 60' foot time was nothing to be proud of due to a serious lack of traction. (Probably helped that Saginaw live another day)
I'm not trying to say that its the greatest thing since sliced bread, but they're not as bad as poeple think. Especially for street performance.
There are a lot of engines that people still build these days that are not the greatest for peformance yet they are still getting screwed together everyday. (aka 326, 350 Pontiac; 350, 403 Olds; nailhead Buick; flathead and "Y" block Fords, etc)
The biggest thing to remember is that no matter how fast you make it, someone will always be faster. Also, be realistic with your goals. As I mentioned earlier, all engines have limitations, especially when you are on a budget. I'm not trying to bash anyone's ride in any way but for some of the folks that are learning, there are MANY other areas to learn about other than what engine should I build for a bit more power? If a 305 is what you already have, go ahead and tinker, it could be the money that wasn't spent on hunting a 350 core that helped pay for your _. (fill in the blank) Another engine can always be built at a later date as true "gearheads" are never really finished with a project. This is all just my opinion of course, I am sure many other may think differently.