Get your priorities figured out.
Are you ever going to sell it? Are you selling it next year?
How original is it now and what else are you doing to it?
Why did you buy/get it?
Which is generally less important than How do you want to use it?
Get that figured out and you can answer your questions yourself, and that will be far more satisfactory than what other people think.
Then you can check the last one which is: How much money and time can you and will you put into it?
340 will get you bragging rights, and easier to obtain 1/4 mile performance. It may gain sales value if done well (both internally and with the entire drivetrain and install) But nobody wants someone elses 1/2 done, or badly done hot rodding project. A lot of people won't even want you hot rod project - they have their own vision of cool.
318 is an easier driver, good milage, good throttle response at street speeds and can pick up e.t. with some bolt on goodies like a 4 bbl, if you actually will run in in the 1/4 mile. Otherwise its mostly bragging right. Hot rod it wrong, and it won't be as nice to drive, and see comments about 340.
