As others said, the only difference is the bore size, stroke length is the same. All your bearings will fit just fine.
The head gaskets are in fact different! The 305 head gasket will stick out into the bore on a 350 and would be bad JuJu! However, you can get away with a 350 head gasket on a 305.
Should be no problem finding heads and manifolds on CL.
If you are going to ditch the TBI, you will need a different distributor. What you want is a plain jain 4 pin HEI. You only need one 10 gauge, 12V wire to make it work.
Very easy to find on any later 70's vehicle. Beware that some early 80's cars and half tons already had a basic computer and the distributor is a little different.
When shopping heads and manifolds keep in mind that Chevy changed to bolt angle on the 4 inner most holes for the intake manifold in 1987. Not a big problem but just so you don't end up with mis-matched parts.