Yes, as far as I know, you can put the 305 crank in the 350. They've got the same stroke. And I believe the journal size in both is the same. But no, you can't put the 350 heads on the 305, without drastically reducing the already low compression of the 305.
If your cylinders are okay, you got lucky. Check the bottom of the skirts carefully to make sure nothing broke through to the water jacket. Even if some compromise there is found, you can have it welded. Yes, iron blocks can be welded.
I'd upgrade the rods though. 305 rods aren't known for their strength. You can get Eagle forged 5140 rods for just a few hundred bucks, and they're far superior to stock cast rods. Then use that crank in the 350 after your machinist checks both out.
Also, plan on overbored pistons. Just to clean up the bores it might be necessary.
Stock 350 heads are fine, but 305 heads would raise the 350's compression too high. So either have a valve job done on the 350 factory heads, or put your money into a vastly superior set of $499 Vortec heads...which will also mean a different intake, but they're worth every penny.
