will they fit? What size are the ports and intake/exhaust? Woul dI be better off getting new "Vortec 23* fast burn heads, aluminum" and getting soem 1.6 self aligning rr's?

 

The L98 Aluminum heads will work well with your 305. But the GMPP Fastburn heads will work a lot better.

 

They will fit but you will loose compression. The 305 heads have 58cc chambers while the 350s have 64cc chambers. The valves on the 350 are the same as the 305, 1.94/1.50. The 350 heads have 160cc intake runners.
As far as the Vortec, they will bot up to the block but you will have to use a Vortec intake. The intake manifold bolts are positioned different from the stock L98 heads. Now, the Vortec heads have the same valve size as the L98 heads, but outflow them due to the intake port design.
Remember that by increasing the intake port volume too much, it will affect velocity and therefore you could be trading low end torque for mid-high end horsepower.
Later,
Ozzy