The main modification that needs to be done is to the cylinder heads themselves. The 305 Vortec heads have a different bolt pattern than the 350 block, so the head bolt holes will need to be drilled and tapped to match the block.
Additionally, the intake manifold and exhaust manifolds will need to be modified to fit the new heads. The intake manifold will need to be ported to match the new head ports, and the exhaust manifolds will need to be modified or replaced to fit the new exhaust port locations.
Finally, the camshaft may need to be changed to match the new heads. The camshaft that was designed for the 305 Vortec heads may not be compatible with the 350 block, so it may be necessary to install a new camshaft that is designed for the 350 block.
Overall, it is possible to install 305 Vortec heads on a 350 block, but it is not a simple bolt-on swap. Some modifications are required to make the heads fit properly.
