Probably going to need more information and some pictures. Has it ever been bored? Has the block ever been decked? Are the main caps there and are they correct for the engine? How has it been stored, what are the dimensions of the crank (mains and rods). This is all going to have an impact on value. If it is virgin (not bored out, crank not cut) and everything is in good order, then it could be work anywhere from $700 to $1500 depending on your area and finding someone who wants it.
The 340 was/is a great engine however it isnt magical unless you have a factory 340 car and are restoring it. Lots of choices out there now to include re-manufactured 340's. If you are building a car and don't have a specific need for a 340, there are lots of 360's out there that can be had for almost nothing. As far as building power, you can do that with pretty much anything to a point. Point being, you need someone who needs a 340 for a specific purpose to maximize its value; that said you are going to need the specifics I mentioned above. Buying a vintage engine that has been taken apart by someone else and stored for a long time is always a risk.
Post as many pictures as you can (block markings, top, bottom, etc), get some measurements and post them.
