The heads will physically bolt onto the block with no mods.
There are at least 2 potential problems though. First, the chambers on the 350 heads are larger than the 305 chambers, so with all else being equal, you'll lower the static compression ratio considerably. Also, the ports in the 350 heads are larger than the 305 heads, so at low rpm's, you may have insufficient vacuum to facilitate good cylinder filling. Secondly, the 305 cylinder bore is 3.736" as compared with 4.000" for the 350 bore, so the valves will be severely shrouded, preventing a good "kernel" formation coming past the valves and resulting in insufficient cylinder filling. If the 350 heads have large valves, it may also be possible to crash a valve into the block deck with a fairly high lift cam. maybe not likely, but possible.

I would only do this if I were absolutely flat broke and needed to get a motor running to have a ride to work. I'd mount the bare block up on an engine stand and mount a head on with a couple of bolts. Then I'd swing the motor around with the main caps up so I could see down the bore and look at the valves. I'd have a buddy push down on the valve stems with a block of wood while I eyeballed the valve/deck interface with a flashlight to check for interference. If all the valves cleared, I'd screw the thing together and run it until I could do better.