In need of an education..340 vs 360.

The shorter stroke of a 340 will make less torque then a 360 all else being equal. All else being equal the 360 is the better engine because of the longer stroke.

If you pull a valve cover that will give you the casting number of the head. But that will not tell you if its big valve or not. '71 was big valve so it if has original heads then big valve.

The best thing you can do to that 340 is swap in some lighter pistons. The stock 340 pistons are extremely heavy. EQ heads will give you quench but would require thick head gaskets with stock pistons because they stick up above the deck.

Bet you could sell that 340 for some serious money and then go build a killer 360 or stroker.