I'll throw my 2 cents in too. Out of all the small blocks, the 350 tends to be the cheapest by a few dollars because more kits are sold for them. I would stay away from a parts store remanufactured long block. I have seen these engines bored as much as .080 over and they will grind down valve stems and install liners in the guides in order to not use new valves, crankshafts are built up and reground too. The rebuilders will do anything they can to use the cores that are turned in to them. The new engines from GM are an good option, but if you have a good local machine shop and the core 350 is ok this would be a good route to take. With the right radiator the 350 should run fine. In my experence (10 yrs automotive machinist) I haven't seen 305s with thick cylnder walls, they use a diffrent core in the water jacket when the block is cast, the nomal thickness is roughly the same as a 350. The diffrence in temp mentioned above is probally due to the 350 making more power. Power = Heat. Just my 2 cents.
Sean
