Is that the original engine from the car? If that's the case, finding usable a garden variety or a later hydraulic roller cam 318 for cheap (if that's possible at your location) and using the 340 intake, carb, and exhaust manifolds may be the best option for now, providing you could get into one for minimal investment. That would allow you to use and enjoy the car some while buying time for the 340 until you can go through it and check things out or send it on out for rebuild. At least enough to make sure that there aren't greater issues at hand or that if any moly assembly lube has been used that it hasn't drawn moisture and corroded the crank journals and that everything is at proper torque spec. The real question that needs to be answered here is "Was the engine gone through and put back together with the intention of the prior owner to keep the car and get it back together and if so so did they do it with enough competence for it to run reliably without risking major damage?" "And what was wrong that merited them going through it to start with?" A quick indicator is seeing if the rocker arms and shafts have been changed or if they are the original ones off the engine with the rocker shaft holes egged out on the bottom. If it's the original shaft and it's off of the head, pull the end bolts and spray some carb cleaner from one end to the other to see if it has been cleaned inside. Look around at the places dirt and sludge hide and see how clean things are. That's going to let you know a whole lot about what you can expect. And as noted, if you do elect to get into it just to clean and reassemble then a dial bore gauge and some Plastigauge will let you know if you need to wait until you can machine it or go ahead and run it. If this were a 318 or 360, I'd give give it the bare minimum and let the budget build fly. But all the checking, cleaning, gaskets, and probably a usable donor 318 is going to be less than replacing a 340 block.
