How far gone is the original 340 anyway? Cracked block? Spray metal welding can work miracles, well that and sleeves. Which, if you can save the original block, for a reasonable price, I say do it. Then prep it for resale/storage. Than go grab a crate motor of your choice for ramming around in. Like the commercial says "why pick one when you can have both".
Anyone every wonder how they "cheater port" factory iron for racing classes. I'll tell yah, one port the head as normal, two spray metal weld a small layer back in the port and presto as cast finish again. I'm buying myself one off of ebay, because there are so many uses for it. Harding metal surfaces on homemade tools etc. cool stuff.